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 Data Manipulation (DML) StatementsBuilt-in Scalar Functions 

ON EXTERNAL [DATA SOURCE]

With ON EXTERNAL [DATA SOURCE], the SQL statement is executed in a separate connection to the same or another database, possibly even on another server.If connection_string is NULL or “''” (empty string), the entire ON EXTERNAL [DATA SOURCE] clause is considered absent, and the statement is executed against the current database.

Connection Pooling

  • External connections made by statements WITH COMMON TRANSACTION (the default) will remain open until the current transaction ends.They can be reused by subsequent calls to EXECUTE STATEMENT, but only if connection_string is identical, including case

  • External connections made by statements WITH AUTONOMOUS TRANSACTION are closed as soon as the statement has been executed

  • Statements using WITH AUTONOMOUS TRANSACTION can and will re-use connections that were opened earlier by statements WITH COMMON TRANSACTION.If this happens, the reused connection will be left open after the statement has been executed.(It must be, because it has at least one active transaction!)

Transaction Pooling

  • If WITH COMMON TRANSACTION is in effect, transactions will be reused as much as possible.They will be committed or rolled back together with the current transaction

  • If WITH AUTONOMOUS TRANSACTION is specified, a fresh transaction will always be started for the statement.This transaction will be committed or rolled back immediately after the statement’s execution

Exception Handling

When ON EXTERNAL is used, the extra connection is always made via a so-called external provider, even if the connection is to the current database.One of the consequences is that exceptions cannot be caught in the usual way.Every exception caused by the statement is wrapped in either an eds_connection or an eds_statement error.To catch them in your PSQL code, you have to use WHEN GDSCODE eds_connection, WHEN GDSCODE eds_statement or WHEN ANY.

Note

Without ON EXTERNAL, exceptions are caught in the usual way, even if an extra connection is made to the current database.
Miscellaneous Notes

  • The character set used for the external connection is the same as that for the current connection

  • Two-phase commits are not supported

AS USER, PASSWORD and ROLE

The optional AS USER, PASSWORD and ROLE clauses allow specification of which user will execute the SQL statement and with which role.The method of user login, and whether a separate connection is opened, depends on the presence and values of the ON EXTERNAL [DATA SOURCE], AS USER, PASSWORD and ROLE clauses:

  • If ON EXTERNAL is present, a new connection is always opened, and:

    • If at least one of AS USER, PASSWORD and ROLE is present, native authentication is attempted with the given parameter values (locally or remotely, depending on connection_string).No defaults are used for missing parameters

    • If all three are absent, and connection_string contains no hostname, then the new connection is established on the local server with the same user and role as the current connection.The term 'local' means “on the same machine as the server” here.This is not necessarily the location of the client

    • If all three are absent, and connection_string contains a hostname, then trusted authentication is attempted on the remote host (again, 'remote' from the perspective of the server).If this succeeds, the remote operating system will provide the username (usually the operating system account under which the Firebird process runs)

  • If ON EXTERNAL is absent:

    • If at least one of AS USER, PASSWORD and ROLE is present, a new connection to the current database is opened with the supplied parameter values.No defaults are used for missing parameters

    • If all three are absent, the statement is executed within the current connection

Note

If a parameter value is NULL or “''” (empty string), the entire parameter is considered absent.Additionally, AS USER is considered absent if its value is equal to CURRENT_USER, and ROLE if it is the same as CURRENT_ROLE.

Caveats with EXECUTE STATEMENT

  1. There is no way to validate the syntax of the enclosed statement

  2. There are no dependency checks to discover whether tables or columns have been dropped

  3. Execution is considerably slower than when the same statements are executed directly as PSQL code

  4. Return values are strictly checked for data type to avoid unpredictable type-casting exceptions.For example, the string '1234' would convert to an integer, 1234, but 'abc' would give a conversion error

All in all, this feature is meant to be used cautiously, and you should always take the caveats into account.If you can achieve the same result with PSQL and/or DSQL, it will almost always be preferable.

See also

[fblangref50-psql-forexec]

FOR SELECT

Loops row-by-row through a query result set

Syntax
[label:]
FOR <select_stmt> [AS CURSOR cursor_name]
  DO <compound_statement>
Table 1. FOR SELECT Statement Parameters
Argument Description

label

Optional label for LEAVE and CONTINUE.Follows the rules for identifiers.

select_stmt

SELECT statement

cursor_name

Cursor name.It must be unique among cursor names in the PSQL module (stored procedure, stored function, trigger or PSQL block)

compound_statement

A single statement, or statements wrapped in BEGIN…​END, that performs all the processing for this FOR loop

The FOR SELECT statement

  • retrieves each row sequentially from the result set, and executes the statement or block of statements for each row.In each iteration of the loop, the field values of the current row are copied into pre-declared variables.

    Including the AS CURSOR clause enables positioned deletes and updates to be performed — see notes below

  • can embed other FOR SELECT statements

  • can contain named parameters that must be previously declared in the DECLARE VARIABLE statement or exist as input or output parameters of the procedure

  • requires an INTO clause at the end of the SELECT …​ FROM …​ specification if AS CURSOR is absentIn each iteration of the loop, the field values of the current row are copied to the list of variables specified in the INTO clause.The loop repeats until all rows are retrieved, after which it terminates

  • can be terminated before all rows are retrieved by using a BREAK, LEAVE or EXIT statement

The Undeclared Cursor

The optional AS CURSOR clause surfaces the result set of the FOR SELECT structure as an undeclared, named cursor that can be operated on using the WHERE CURRENT OF clause inside the statement or block following the DO command, to delete or update the current row before execution moves to the next row.In addition, it is possible to use the cursor name as a record variable (similar to OLD and NEW in triggers), allowing access to the columns of the result set (i.e. cursor_name.columnname).

Rules for Cursor Variables

  • When accessing a cursor variable in a DML statement, the colon prefix can be added before the cursor name (i.e. :cursor_name.columnname) for disambiguation, similar to variables.

    The cursor variable can be referenced without colon prefix, but in that case, depending on the scope of the contexts in the statement, the name may resolve in the statement context instead of to the cursor (e.g. you select from a table with the same name as the cursor).

  • Cursor variables are read-only

  • In a FOR SELECT statement without an AS CURSOR clause, you must use the INTO clause.If an AS CURSOR clause is specified, the INTO clause is allowed, but optional;you can access the fields through the cursor instead.

  • Reading from a cursor variable returns the current field values.This means that an UPDATE statement (with a WHERE CURRENT OF clause) will update not only the table, but also the fields in the cursor variable for subsequent reads.Executing a DELETE statement (with a WHERE CURRENT OF clause) will set all fields in the cursor variable to NULL for subsequent reads

Other points to take into account regarding undeclared cursors:

  1. The OPEN, FETCH and CLOSE statements cannot be applied to a cursor surfaced by the AS CURSOR clause

  2. The cursor_name argument associated with an AS CURSOR clause must not clash with any names created by DECLARE VARIABLE or DECLARE CURSOR statements at the top of the module body, nor with any other cursors surfaced by an AS CURSOR clause

  3. The optional FOR UPDATE clause in the SELECT statement is not required for a positioned update

Examples using FOR SELECT

  1. A simple loop through query results:

    CREATE PROCEDURE SHOWNUMS
RETURNS (
  AA INTEGER,
  BB INTEGER,
  SM INTEGER,
  DF INTEGER)
AS
BEGIN
  FOR SELECT DISTINCT A, B
      FROM NUMBERS
    ORDER BY A, B
    INTO AA, BB
  DO
  BEGIN
    SM = AA + BB;
    DF = AA - BB;
    SUSPEND;
  END
END

  2. Nested FOR SELECT loop:

    CREATE PROCEDURE RELFIELDS
RETURNS (
  RELATION CHAR(32),
  POS INTEGER,
  FIELD CHAR(32))
AS
BEGIN
  FOR SELECT RDB$RELATION_NAME
      FROM RDB$RELATIONS
      ORDER BY 1
      INTO :RELATION
  DO
  BEGIN
    FOR SELECT
          RDB$FIELD_POSITION + 1,
          RDB$FIELD_NAME
        FROM RDB$RELATION_FIELDS
        WHERE
          RDB$RELATION_NAME = :RELATION
        ORDER BY RDB$FIELD_POSITION
        INTO :POS, :FIELD
    DO
    BEGIN
      IF (POS = 2) THEN
        RELATION = ' "';

      SUSPEND;
    END
  END
END
    Tip
    		 Instead of nesting statements, this is generally better solved by using a single statements with a join.

  3. Using the AS CURSOR clause to surface a cursor for the positioned delete of a record:

    CREATE PROCEDURE DELTOWN (
  TOWNTODELETE VARCHAR(24))
RETURNS (
  TOWN VARCHAR(24),
  POP INTEGER)
AS
BEGIN
  FOR SELECT TOWN, POP
      FROM TOWNS
      INTO :TOWN, :POP AS CURSOR TCUR
  DO
  BEGIN
    IF (:TOWN = :TOWNTODELETE) THEN
      -- Positional delete
      DELETE FROM TOWNS
      WHERE CURRENT OF TCUR;
    ELSE
      SUSPEND;
  END
END

  4. Using an implicitly declared cursor as a cursor variable

    EXECUTE BLOCK
 RETURNS (o CHAR(63))
AS
BEGIN
  FOR SELECT rdb$relation_name AS name
    FROM rdb$relations AS CURSOR c
  DO
  BEGIN
    o = c.name;
    SUSPEND;
  END
END

  5. Disambiguating cursor variables within queries

    EXECUTE BLOCK
  RETURNS (o1 CHAR(63), o2 CHAR(63))
AS
BEGIN
  FOR SELECT rdb$relation_name
    FROM rdb$relations
    WHERE
      rdb$relation_name = 'RDB$RELATIONS' AS CURSOR c
  DO
  BEGIN
    FOR SELECT
        -- with a prefix resolves to the cursor
        :c.rdb$relation_name x1,
        -- no prefix as an alias for the rdb$relations table
        c.rdb$relation_name x2
      FROM rdb$relations c
      WHERE
        rdb$relation_name = 'RDB$DATABASE' AS CURSOR d
    DO
    BEGIN
      o1 = d.x1;
      o2 = d.x2;
      SUSPEND;
    END
  END
END
See also

[fblangref50-psql-declare-cursor], [fblangref50-psql-break], [fblangref50-psql-leave], [fblangref50-psql-continue], [fblangref50-psql-exit], SELECT, UPDATE, DELETE

FOR EXECUTE STATEMENT

Executes dynamically created SQL statements and loops over its result set

Syntax
[label:]
FOR <execute_statement> DO <compound_statement>
Table 1. FOR EXECUTE STATEMENT Statement Parameters
Argument Description

label

Optional label for LEAVE and CONTINUE.Follows the rules for identifiers.

execute_stmt

An EXECUTE STATEMENT statement

compound_statement

A single statement, or statements wrapped in BEGIN …​ END, that performs all the processing for this FOR loop

The statement FOR EXECUTE STATEMENT is used, in a manner analogous to FOR SELECT, to loop through the result set of a dynamically executed query that returns multiple rows.

FOR EXECUTE STATEMENT Examples

Executing a dynamically constructed SELECT query that returns a data set
CREATE PROCEDURE DynamicSampleThree (
   Q_FIELD_NAME VARCHAR(100),
   Q_TABLE_NAME VARCHAR(100)
) RETURNS(
  LINE VARCHAR(32000)
)
AS
  DECLARE VARIABLE P_ONE_LINE VARCHAR(100);
BEGIN
  LINE = '';
  FOR
    EXECUTE STATEMENT
      'SELECT T1.' || :Q_FIELD_NAME ||
      ' FROM ' || :Q_TABLE_NAME || ' T1 '
    INTO :P_ONE_LINE
  DO
    IF (:P_ONE_LINE IS NOT NULL) THEN
      LINE = :LINE || :P_ONE_LINE || ' ';
  SUSPEND;
END
See also

[fblangref50-psql-execstmt], [fblangref50-psql-break], [fblangref50-psql-leave], [fblangref50-psql-continue]

OPEN

Opens a declared cursor

Syntax
OPEN cursor_name;
Table 1. OPEN Statement Parameter
Argument Description

cursor_name

Cursor name.A cursor with this name must be previously declared with a DECLARE CURSOR statement

An OPEN statement opens a previously declared cursor, executes its declared SELECT statement, and makes the first record of the result data set ready to fetch.OPEN can be applied only to cursors previously declared in a [fblangref50-psql-declare-cursor] statement.

Note

If the SELECT statement of the cursor has parameters, they must be declared as local variables, or input or output parameters before the cursor is declared.When the cursor is opened, the parameter is assigned the current value of the variable.

OPEN Examples

  1. Using the OPEN statement:

    SET TERM ^;

CREATE OR ALTER PROCEDURE GET_RELATIONS_NAMES
RETURNS (
  RNAME CHAR(63)
)
AS
  DECLARE C CURSOR FOR (
    SELECT RDB$RELATION_NAME
    FROM RDB$RELATIONS);
BEGIN
  OPEN C;
  WHILE (1 = 1) DO
  BEGIN
    FETCH C INTO :RNAME;
    IF (ROW_COUNT = 0) THEN
      LEAVE;
    SUSPEND;
  END
  CLOSE C;
END^

SET TERM ;^

  2. A collection of scripts for creating views using a PSQL block with named cursors:

    EXECUTE BLOCK
RETURNS (
  SCRIPT BLOB SUB_TYPE TEXT)
AS
  DECLARE VARIABLE FIELDS VARCHAR(8191);
  DECLARE VARIABLE FIELD_NAME TYPE OF RDB$FIELD_NAME;
  DECLARE VARIABLE RELATION RDB$RELATION_NAME;
  DECLARE VARIABLE SOURCE TYPE OF COLUMN RDB$RELATIONS.RDB$VIEW_SOURCE;
  -- named cursor
  DECLARE VARIABLE CUR_R CURSOR FOR (
    SELECT
      RDB$RELATION_NAME,
      RDB$VIEW_SOURCE
    FROM
      RDB$RELATIONS
    WHERE
      RDB$VIEW_SOURCE IS NOT NULL);
  -- named cursor with local variable
  DECLARE CUR_F CURSOR FOR (
    SELECT
      RDB$FIELD_NAME
    FROM
      RDB$RELATION_FIELDS
    WHERE
      -- Important! The variable has to be declared earlier
      RDB$RELATION_NAME = :RELATION);
BEGIN
  OPEN CUR_R;
  WHILE (1 = 1) DO
  BEGIN
    FETCH CUR_R
      INTO :RELATION, :SOURCE;
    IF (ROW_COUNT = 0) THEN
      LEAVE;

    FIELDS = NULL;
    -- The CUR_F cursor will use
    -- variable value of RELATION initialized above
    OPEN CUR_F;
    WHILE (1 = 1) DO
    BEGIN
      FETCH CUR_F
        INTO :FIELD_NAME;
      IF (ROW_COUNT = 0) THEN
        LEAVE;
      IF (FIELDS IS NULL) THEN
        FIELDS = TRIM(FIELD_NAME);
      ELSE
        FIELDS = FIELDS || ', ' || TRIM(FIELD_NAME);
    END
    CLOSE CUR_F;

    SCRIPT = 'CREATE VIEW ' || RELATION;

    IF (FIELDS IS NOT NULL) THEN
      SCRIPT = SCRIPT || ' (' || FIELDS || ')';

    SCRIPT = SCRIPT || ' AS ' || ASCII_CHAR(13);
    SCRIPT = SCRIPT || SOURCE;

    SUSPEND;
  END
  CLOSE CUR_R;
END
See also

[fblangref50-psql-declare-cursor], [fblangref50-psql-fetch], [fblangref50-psql-close]

FETCH

Fetches a record from a cursor

Syntax
FETCH [<fetch_scroll> FROM] cursor_name
  [INTO [:]varname [, [:]varname ...]];

<fetch_scroll> ::=
    NEXT | PRIOR | FIRST | LAST
  | RELATIVE n | ABSOLUTE n
Table 1. FETCH Statement Parameters
Argument Description

cursor_name

Cursor name.A cursor with this name must be previously declared with a DECLARE …​ CURSOR statement and opened by an OPEN statement.

varname

Variable name

n

Integer expression for the number of rows

The FETCH statement fetches the next row from the result set of the cursor and assigns the column values to PSQL variables.The FETCH statement can be used only with a cursor declared with the [fblangref50-psql-declare-cursor] statement.

Using the optional fetch_scroll part of the FETCH statement, you can specify in which direction and how many rows to advance the cursor position.The NEXT fetch option can be used for scrollable and forward-only cursors.Other fetch options are only supported for scrollable cursors.

The Fetch Options
NEXT

moves the cursor one row forward;this is the default

PRIOR

moves the cursor one record back

FIRST

moves the cursor to the first record.

LAST

moves the cursor to the last record

RELATIVE n

moves the cursor n rows from the current position;positive numbers move forward, negative numbers move backwards;using zero (0) will not move the cursor, and ROW_COUNT will be set to zero as no new row was fetched.

ABSOLUTE n

moves the cursor to the specified row;n is an integer expression, where 1 indicates the first row.For negative values, the absolute position is taken from the end of the result set, so -1 indicates the last row, -2 the second to last row, etc.A value of zero (0) will position before the first row.

The optional INTO clause gets data from the current row of the cursor and loads them into PSQL variables.If a fetch moves beyond the bounds of the result set, the variables will be set to NULL.

It is also possible to use the cursor name as a variable of a record type (similar to OLD and NEW in triggers), allowing access to the columns of the result set (i.e. cursor_name.columnname).

Rules for Cursor Variables

  • When accessing a cursor variable in a DML statement, the colon prefix can be added before the cursor name (i.e. :cursor_name.columnname) for disambiguation, similar to variables.

    The cursor variable can be referenced without colon prefix, but in that case, depending on the scope of the contexts in the statement, the name may resolve in the statement context instead of to the cursor (e.g. you select from a table with the same name as the cursor).

  • Cursor variables are read-only

  • In a FOR SELECT statement without an AS CURSOR clause, you must use the INTO clause.If an AS CURSOR clause is specified, the INTO clause is allowed, but optional;you can access the fields through the cursor instead.

  • Reading from a cursor variable returns the current field values.This means that an UPDATE statement (with a WHERE CURRENT OF clause) will update not only the table, but also the fields in the cursor variable for subsequent reads.Executing a DELETE statement (with a WHERE CURRENT OF clause) will set all fields in the cursor variable to NULL for subsequent reads

  • When the cursor is not positioned on a row — it is positioned before the first row, or after the last row — attempts to read from the cursor variable will result in error “Cursor cursor_name is not positioned in a valid record”

For checking whether all the rows of the result set have been fetched, the context variable ROW_COUNT returns the number of rows fetched by the statement.If a record was fetched, then ROW_COUNT is one (1), otherwise zero (0).

FETCH Examples

  1. Using the FETCH statement:

    CREATE OR ALTER PROCEDURE GET_RELATIONS_NAMES
  RETURNS (RNAME CHAR(63))
AS
  DECLARE C CURSOR FOR (
    SELECT RDB$RELATION_NAME
    FROM RDB$RELATIONS);
BEGIN
  OPEN C;
  WHILE (1 = 1) DO
  BEGIN
    FETCH C INTO RNAME;
    IF (ROW_COUNT = 0) THEN
      LEAVE;
    SUSPEND;
  END
  CLOSE C;
END

  2. Using the FETCH statement with nested cursors:

    EXECUTE BLOCK
  RETURNS (SCRIPT BLOB SUB_TYPE TEXT)
AS
  DECLARE VARIABLE FIELDS VARCHAR (8191);
  DECLARE VARIABLE FIELD_NAME TYPE OF RDB$FIELD_NAME;
  DECLARE VARIABLE RELATION RDB$RELATION_NAME;
  DECLARE VARIABLE SRC TYPE OF COLUMN RDB$RELATIONS.RDB$VIEW_SOURCE;
  -- Named cursor declaration
  DECLARE VARIABLE CUR_R CURSOR FOR (
    SELECT
      RDB$RELATION_NAME,
      RDB$VIEW_SOURCE
    FROM RDB$RELATIONS
    WHERE RDB$VIEW_SOURCE IS NOT NULL);
  -- Declaring a named cursor in which
  -- a local variable is used
  DECLARE CUR_F CURSOR FOR (
    SELECT RDB$FIELD_NAME
    FROM RDB$RELATION_FIELDS
    WHERE
    -- the variable must be declared earlier
      RDB$RELATION_NAME =: RELATION);
BEGIN
  OPEN CUR_R;
  WHILE (1 = 1) DO
  BEGIN
    FETCH CUR_R INTO RELATION, SRC;
    IF (ROW_COUNT = 0) THEN
      LEAVE;
    FIELDS = NULL;
    -- Cursor CUR_F will use the value
    -- the RELATION variable initialized above
    OPEN CUR_F;
    WHILE (1 = 1) DO
    BEGIN
      FETCH CUR_F INTO FIELD_NAME;
      IF (ROW_COUNT = 0) THEN
        LEAVE;
      IF (FIELDS IS NULL) THEN
        FIELDS = TRIM (FIELD_NAME);
      ELSE
        FIELDS = FIELDS || ',' || TRIM(FIELD_NAME);
    END
    CLOSE CUR_F;
    SCRIPT = 'CREATE VIEW' || RELATION;
    IF (FIELDS IS NOT NULL) THEN
      SCRIPT = SCRIPT || '(' || FIELDS || ')' ;
    SCRIPT = SCRIPT || 'AS' || ASCII_CHAR (13);
    SCRIPT = SCRIPT || SRC;
    SUSPEND;
  END
  CLOSE CUR_R;
EN

  3. An example of using the FETCH statement with a scrollable cursor

EXECUTE BLOCK
  RETURNS (N INT, RNAME CHAR (63))
AS
  DECLARE C SCROLL CURSOR FOR (
    SELECT
      ROW_NUMBER() OVER (ORDER BY RDB$RELATION_NAME) AS N,
      RDB$RELATION_NAME
    FROM RDB$RELATIONS
    ORDER BY RDB$RELATION_NAME);
BEGIN
  OPEN C;
  -- move to the first record (N = 1)
  FETCH FIRST FROM C;
  RNAME = C.RDB$RELATION_NAME;
  N = C.N;
  SUSPEND;
  -- move 1 record forward (N = 2)
  FETCH NEXT FROM C;
  RNAME = C.RDB$RELATION_NAME;
  N = C.N;
  SUSPEND;
  -- move to the fifth record (N = 5)
  FETCH ABSOLUTE 5 FROM C;
  RNAME = C.RDB$RELATION_NAME;
  N = C.N;
  SUSPEND;
  -- move 1 record backward (N = 4)
  FETCH PRIOR FROM C;
  RNAME = C.RDB$RELATION_NAME;
  N = C.N;
  SUSPEND;
  -- move 3 records forward (N = 7)
  FETCH RELATIVE 3 FROM C;
  RNAME = C.RDB$RELATION_NAME;
  N = C.N;
  SUSPEND;
  -- move back 5 records (N = 2)
  FETCH RELATIVE -5 FROM C;
  RNAME = C.RDB$RELATION_NAME;
  N = C.N;
  SUSPEND;
  -- move to the first record (N = 1)
  FETCH FIRST FROM C;
  RNAME = C.RDB$RELATION_NAME;
  N = C.N;
  SUSPEND;
  -- move to the last entry
  FETCH LAST FROM C;
  RNAME = C.RDB$RELATION_NAME;
  N = C.N;
  SUSPEND;
  CLOSE C;
END
See also

[fblangref50-psql-declare-cursor], [fblangref50-psql-open], [fblangref50-psql-close]

Management Statements in PSQL

Management statement are allowed in PSQL modules (triggers, procedures, functions and PSQL blocks), which is especially helpful for applications that need management statements to be executed at the start of a session, specifically in ON CONNECT triggers.

The management statements permitted in PSQL are:

Example of Management Statements in PSQL

create or alter trigger on_connect on connect
as
begin
    set bind of decfloat to double precision;
    set time zone 'America/Sao_Paulo';
end
Caution

Although useful as a workaround, using ON CONNECT triggers to configure bind and time zone is usually not the right approach.Alternatives are handling this through DefaultTimeZone in firebird.conf and DataTypeCompatibility in firebird.conf or databases.conf, or isc_dpb_session_time_zone or isc_dpb_set_bind in the DPB.
See also

Management Statements

CLOSE

Closes a declared cursor

Syntax
CLOSE cursor_name;
Table 1. CLOSE Statement Parameter
Argument Description

cursor_name

Cursor name.A cursor with this name must be previously declared with a DECLARE …​ CURSOR statement and opened by an OPEN statement

A CLOSE statement closes an open cursor.Only a cursor that was declared with [fblangref50-psql-declare-cursor] can be closed with a CLOSE statement.Any cursors that are still open will be automatically closed after the module code completes execution.

CLOSE Examples

See [fblangref50-psql-fetch-exmpl]

See also

[fblangref50-psql-declare-cursor], [fblangref50-psql-open], [fblangref50-psql-fetch]

IN AUTONOMOUS TRANSACTION

Executes a statement or a block of statements in an autonomous transaction

Syntax
IN AUTONOMOUS TRANSACTION DO <compound_statement>
Table 1. IN AUTONOMOUS TRANSACTION Statement Parameter
Argument Description

compound_statement

A single statement, or statements wrapped in BEGIN …​ END

The IN AUTONOMOUS TRANSACTION statement enables execution of a statement or a block of statements in an autonomous transaction.Code running in an autonomous transaction will be committed right after its successful execution, regardless of the status of its parent transaction.This can be used when certain operations must not be rolled back, even if an error occurs in the parent transaction.

An autonomous transaction has the same isolation level as its parent transaction.Any exception that is thrown in the block of the autonomous transaction code will result in the autonomous transaction being rolled back and all changes made will be undone.If the code executes successfully, the autonomous transaction will be committed.

IN AUTONOMOUS TRANSACTION Examples

Using an autonomous transaction in a trigger for the database ON CONNECT event, to log all connection attempts, including those that failed:

CREATE TRIGGER TR_CONNECT ON CONNECT
AS
BEGIN
  -- Logging all attempts to connect to the database
  IN AUTONOMOUS TRANSACTION DO
    INSERT INTO LOG(MSG)
    VALUES ('USER ' || CURRENT_USER || ' CONNECTS.');
  IF (EXISTS(SELECT *
             FROM BLOCKED_USERS
             WHERE USERNAME = CURRENT_USER)) THEN
  BEGIN
    -- Logging that the attempt to connect
    -- to the database failed and sending
    -- a message about the event
    IN AUTONOMOUS TRANSACTION DO
    BEGIN
      INSERT INTO LOG(MSG)
      VALUES ('USER ' || CURRENT_USER || ' REFUSED.');
      POST_EVENT 'CONNECTION ATTEMPT BY BLOCKED USER!';
    END
    -- now calling an exception
    EXCEPTION EX_BADUSER;
  END
END
See also

Transaction Control

POST_EVENT

Posts an event for notification to registered clients on commit

Syntax
POST_EVENT event_name;
Table 1. POST_EVENT Statement Parameter
Argument Description

event_name

Event name (message) limited to 127 bytes

The POST_EVENT statement notifies the event manager about the event, which saves it to an event table.When the transaction is committed, the event manager notifies applications that have registered their interest in the event.

The event name can be a code, or a short message: the choice is open as it is a string of up to 127 bytes.Keep in mind that the application listening for an event must use the exact event name when registering.

The content of the string can be a string literal, a variable or any valid SQL expression that resolves to a string.

POST_EVENT Examples

Notifying the listening applications about inserting a record into the SALES table:

CREATE TRIGGER POST_NEW_ORDER FOR SALES
ACTIVE AFTER INSERT POSITION 0
AS
BEGIN
  POST_EVENT 'new_order';
END

RETURN

Returns a value from a stored function

Syntax
RETURN value;
Table 1. RETURN Statement Parameter
Argument Description

value

Expression with the value to return;Can be any expression type-compatible with the return type of the function

The RETURN statement ends the execution of a function and returns the value of the expression value.

RETURN can only be used in PSQL functions (stored functions and local sub-functions).

RETURN Examples

See CREATE FUNCTION Examples

DECLARE VARIABLE

Declares a local variable

Syntax
DECLARE [VARIABLE] varname
  <domain_or_non_array_type> [NOT NULL] [COLLATE collation]
  [{DEFAULT | = } <initvalue>];

<domain_or_non_array_type> ::=
  !! See Scalar Data Types Syntax !!

<initvalue> ::= <literal> | <context_var>
Table 1. DECLARE VARIABLE Statement Parameters
Argument Description

varname

Name of the local variable

collation

Collation

initvalue

Initial value for this variable

literal

Literal of a type compatible with the type of the local variable

context_var

Any context variable whose type is compatible with the type of the local variable

The statement DECLARE [VARIABLE] is used for declaring a local variable.One DECLARE [VARIABLE] statement is required for each local variable.Any number of DECLARE [VARIABLE] statements can be included and in any order.The name of a local variable must be unique among the names of local variables and input and output parameters declared for the module.

A special case of DECLARE [VARIABLE] — declaring cursors — is covered separately in [fblangref50-psql-declare-cursor]

Data Type for Variables

A local variable can be of any SQL type other than an array.

  • A domain name can be specified as the type;the variable will inherit all of its attributes.

  • If the TYPE OF domain clause is used instead, the variable will inherit only the domain’s data type, and, if applicable, its character set and collation attributes.Any default value or constraints such as NOT NULL or CHECK constraints are not inherited.

  • If the TYPE OF COLUMN relation.column option is used to “borrow” from a column in a table or view, the variable will inherit only the column’s data type, and, if applicable, its character set and collation attributes.Any other attributes are ignored.

NOT NULL Constraint

For local variables, you can specify the NOT NULL constraint, disallowing NULL values for the variable.If a domain has been specified as the data type and the domain already has the NOT NULL constraint, the declaration is unnecessary.For other forms, including use of a domain that is nullable, the NOT NULL constraint can be included if needed.

CHARACTER SET and COLLATE clauses

Unless specified, the character set and collation of a string variable will be the database defaults.A CHARACTER SET clause can be specified to handle string data that needs a different character set.A valid collation (COLLATE clause) can also be included, with or without the character set clause.

Initializing a Variable

Local variables are NULL when execution of the module begins.They can be explicitly initialized so that a starting or default value is available when they are first referenced.The initial value can be specified in two ways, DEFAULT <initvalue> and = <initvalue>.The value can be any type-compatible literal or context variable, including NULL.

Tip

Be sure to use this clause for any variables that have a NOT NULL constraint and do not otherwise have a default value available (i.e. inherited from a domain).

Examples of various ways to declare local variables

CREATE OR ALTER PROCEDURE SOME_PROC
AS
  -- Declaring a variable of the INT type
  DECLARE I INT;
  -- Declaring a variable of the INT type that does not allow NULL
  DECLARE VARIABLE J INT NOT NULL;
  -- Declaring a variable of the INT type with the default value of 0
  DECLARE VARIABLE K INT DEFAULT 0;
  -- Declaring a variable of the INT type with the default value of 1
  DECLARE VARIABLE L INT = 1;
  -- Declaring a variable based on the COUNTRYNAME domain
  DECLARE FARM_COUNTRY COUNTRYNAME;
  -- Declaring a variable of the type equal to the COUNTRYNAME domain
  DECLARE FROM_COUNTRY TYPE OF COUNTRYNAME;
  -- Declaring a variable with the type of the CAPITAL column in the COUNTRY table
  DECLARE CAPITAL TYPE OF COLUMN COUNTRY.CAPITAL;
BEGIN
  /* PSQL statements */
END
See also

Data Types and Subtypes, Custom Data Types — Domains, CREATE DOMAIN

DECLARE .. CURSOR

Declares a named cursor

Syntax
DECLARE [VARIABLE] cursor_name
  [[NO] SCROLL] CURSOR
  FOR (<select>);
Table 1. DECLARE …​ CURSOR Statement Parameters
Argument Description

cursor_name

Cursor name

select

SELECT statement

The DECLARE …​ CURSOR …​ FOR statement binds a named cursor to the result set obtained by the SELECT statement specified in the FOR clause.In the body code, the cursor can be opened, used to iterate row-by-row through the result set, and closed.While the cursor is open, the code can perform positioned updates and deletes using the WHERE CURRENT OF in the UPDATE or DELETE statement.

Note

Syntactically, the DECLARE …​ CURSOR statement is a special case of [fblangref50-psql-declare-variable].

Forward-Only and Scrollable Cursors

The cursor can be forward-only (unidirectional) or scrollable.The optional clause SCROLL makes the cursor scrollable, the NO SCROLL clause, forward-only.By default, cursors are forward-only.

Forward-only cursors can — as the name implies — only move forward in the dataset.Forward-only cursors only support the FETCH [NEXT FROM] statement, other fetch options raise an error.Scrollable cursors allow you to move not only forward in the dataset, but also back, as well as N positions relative to the current position.

Warning

Scrollable cursors are materialized as a temporary dataset, as such, they consume additional memory or disk space, so use them only when you really need them.

Cursor Idiosyncrasies

  • The optional FOR UPDATE clause can be included in the SELECT statement, but its absence does not prevent successful execution of a positioned update or delete

  • Care should be taken to ensure that the names of declared cursors do not conflict with any names used subsequently in statements for AS CURSOR clauses

  • If the cursor is needed only to walk the result set, it is nearly always easier and less error-prone to use a FOR SELECT statement with the AS CURSOR clause.Declared cursors must be explicitly opened, used to fetch data, and closed.The context variable ROW_COUNT has to be checked after each fetch and, if its value is zero, the loop has to be terminated.A FOR SELECT statement does this automatically.

    Nevertheless, declared cursors provide a high level of control over sequential events and allow several cursors to be managed in parallel.

  • The SELECT statement may contain parameters. For instance:

    SELECT NAME || :SFX FROM NAMES WHERE NUMBER = :NUM

    Each parameter has to have been declared beforehand as a PSQL variable, or as input or output parameters.When the cursor is opened, the parameter is assigned the current value of the variable.

Warning
Unstable Variables and Cursors

If the value of the PSQL variable used in the SELECT statement of the cursor changes during the execution of the loop, then its new value may — but not always — be used when selecting the next rows.It is better to avoid such situations.If you really need this behaviour, then you should thoroughly test your code and make sure you understand how changes to the variable affect the query results.

Note particularly that the behaviour may depend on the query plan, specifically on the indexes being used.Currently, there are no strict rules for this behaviour, and this may change in future versions of Firebird.

Examples Using Named Cursors

  1. Declaring a named cursor in a trigger.

    CREATE OR ALTER TRIGGER TBU_STOCK
  BEFORE UPDATE ON STOCK
AS
  DECLARE C_COUNTRY CURSOR FOR (
    SELECT
      COUNTRY,
      CAPITAL
    FROM COUNTRY
  );
BEGIN
  /* PSQL statements */
END

  2. Declaring a scrollable cursor

    EXECUTE BLOCK
  RETURNS (
    N INT,
    RNAME CHAR(63))
AS
  - Declaring a scrollable cursor
  DECLARE C SCROLL CURSOR FOR (
    SELECT
      ROW_NUMBER() OVER (ORDER BY RDB$RELATION_NAME) AS N,
      RDB$RELATION_NAME
    FROM RDB$RELATIONS
    ORDER BY RDB$RELATION_NAME);
BEGIN
  / * PSQL statements * /
END

  3. A collection of scripts for creating views with a PSQL block using named cursors.

    EXECUTE BLOCK
RETURNS (
  SCRIPT BLOB SUB_TYPE TEXT)
AS
  DECLARE VARIABLE FIELDS VARCHAR(8191);
  DECLARE VARIABLE FIELD_NAME TYPE OF RDB$FIELD_NAME;
  DECLARE VARIABLE RELATION RDB$RELATION_NAME;
  DECLARE VARIABLE SOURCE TYPE OF COLUMN RDB$RELATIONS.RDB$VIEW_SOURCE;
  DECLARE VARIABLE CUR_R CURSOR FOR (
    SELECT
      RDB$RELATION_NAME,
      RDB$VIEW_SOURCE
    FROM
      RDB$RELATIONS
    WHERE
      RDB$VIEW_SOURCE IS NOT NULL);
  -- Declaring a named cursor where
  -- a local variable is used
  DECLARE CUR_F CURSOR FOR (
    SELECT
      RDB$FIELD_NAME
    FROM
      RDB$RELATION_FIELDS
    WHERE
      -- the variable must be declared earlier
      RDB$RELATION_NAME = :RELATION);
BEGIN
  OPEN CUR_R;
  WHILE (1 = 1) DO
  BEGIN
    FETCH CUR_R
    INTO :RELATION, :SOURCE;
    IF (ROW_COUNT = 0) THEN
      LEAVE;

    FIELDS = NULL;
    -- The CUR_F cursor will use the value
    -- of the RELATION variable initiated above
    OPEN CUR_F;
    WHILE (1 = 1) DO
    BEGIN
      FETCH CUR_F
      INTO :FIELD_NAME;
      IF (ROW_COUNT = 0) THEN
        LEAVE;
      IF (FIELDS IS NULL) THEN
        FIELDS = TRIM(FIELD_NAME);
      ELSE
        FIELDS = FIELDS || ', ' || TRIM(FIELD_NAME);
    END
    CLOSE CUR_F;

    SCRIPT = 'CREATE VIEW ' || RELATION;

    IF (FIELDS IS NOT NULL) THEN
      SCRIPT = SCRIPT || ' (' || FIELDS || ')';

    SCRIPT = SCRIPT || ' AS ' || ASCII_CHAR(13);
    SCRIPT = SCRIPT || SOURCE;

    SUSPEND;
  END
  CLOSE CUR_R;
END
See also

[fblangref50-psql-open], [fblangref50-psql-fetch], [fblangref50-psql-close]

DECLARE FUNCTION

Declares a sub-function

Syntax
<subfunc-forward> ::= <subfunc-header>;

<subfunc-def> ::= <subfunc-header> <psql-module-body>

<subfunc-header>  ::=
  DECLARE FUNCTION subfuncname [ ( [ <in_params> ] ) ]
  RETURNS <domain_or_non_array_type> [COLLATE collation]
  [DETERMINISTIC]

<in_params> ::=
  !! See CREATE FUNCTION Syntax !!

<domain_or_non_array_type> ::=
  !! See Scalar Data Types Syntax !!

<psql-module-body> ::=
  !! See Syntax of Module Body !!
Table 1. DECLARE FUNCTION Statement Parameters
Argument Description

subfuncname

Sub-function name

collation

Collation name

The DECLARE FUNCTION statement declares a sub-function.A sub-function is only visible to the PSQL module that defined the sub-function.

A sub-function can use variables, but not cursors, from its parent module.It can access other routines from its parent modules, including recursive calls to itself.

Sub-functions have a number of restrictions:

  • A sub-function cannot be nested in another subroutine.Subroutines are only supported in top-level PSQL modules (stored procedures, stored functions, triggers and PSQL blocks).This restriction is not enforced by the syntax, but attempts to create nested sub-functions will raise an error “feature is not supported” with detail message “nested sub function”.

  • Currently, a sub-function has no direct access to use cursors from its parent module.

A sub-function can be forward declared to resolve mutual dependencies between subroutines, and must be followed by its actual definition.When a sub-function is forward declared and has parameters with default values, the default values should only be specified in the forward declaration, and should not be repeated in subfunc_def.

Note

Declaring a sub-function with the same name as a stored function will hide that stored function from your module.It will not be possible to call that stored function.
Note

Contrary to DECLARE [VARIABLE], a DECLARE FUNCTION is not terminated by a semicolon.The END of its main BEGIN …​ END block is considered its terminator.

Examples of Sub-Functions

  1. Sub-function within a stored function

    CREATE OR ALTER FUNCTION FUNC1 (n1 INTEGER, n2 INTEGER)
  RETURNS INTEGER
AS
- Subfunction
  DECLARE FUNCTION SUBFUNC (n1 INTEGER, n2 INTEGER)
    RETURNS INTEGER
  AS
  BEGIN
    RETURN n1 + n2;
  END
BEGIN
  RETURN SUBFUNC (n1, n2);
END

  2. Recursive function call

    execute block returns (i integer, o integer)
as
    -- Recursive function without forward declaration.
    declare function fibonacci(n integer) returns integer
    as
    begin
      if (n = 0 or n = 1) then
       return n;
     else
       return fibonacci(n - 1) + fibonacci(n - 2);
    end
begin
  i = 0;

  while (i < 10)
  do
  begin
    o = fibonacci(i);
    suspend;
    i = i + 1;
  end
end
See also

[fblangref50-psql-declproc], CREATE FUNCTION

DECLARE PROCEDURE

Declares a sub-procedure

Syntax
<subproc-forward> ::= <subproc-header>;

<subproc-def> ::= <subproc-header> <psql-module-body>

<subproc-header>  ::=
DECLARE subprocname [ ( [ <in_params> ] ) ]
  [RETURNS (<out_params>)]

<in_params> ::=
  !! See CREATE PROCEDURE Syntax !!

<domain_or_non_array_type> ::=
  !! See Scalar Data Types Syntax !!

<psql-module-body> ::=
  !! See Syntax of Module Body !!
Table 1. DECLARE PROCEDURE Statement Parameters
Argument Description

subprocname

Sub-procedure name

collation

Collation name

The DECLARE PROCEDURE statement declares a sub-procedure.A sub-procedure is only visible to the PSQL module that defined the sub-procedure.

A sub-procedure can use variables, but not cursors, from its parent module.It can access other routines from its parent modules.

Sub-procedures have a number of restrictions:

  • A sub-procedure cannot be nested in another subroutine.Subroutines are only supported in top-level PSQL modules (stored procedures, stored functions, triggers and PSQL blocks).This restriction is not enforced by the syntax, but attempts to create nested sub-procedures will raise an error “feature is not supported” with detail message “nested sub procedure”.

  • Currently, the sub-procedure has no direct access to use cursors from its parent module.

A sub-procedure can be forward declared to resolve mutual dependencies between subroutines, and must be followed by its actual definition.When a sub-procedure is forward declared and has parameters with default values, the default values should only be specified in the forward declaration, and should not be repeated in subproc_def.

Note

Declaring a sub-procedure with the same name as a stored procedure, table or view will hide that stored procedure, table or view from your module.It will not be possible to call that stored procedure, table or view.
Note

Contrary to DECLARE [VARIABLE], a DECLARE PROCEDURE is not terminated by a semicolon.The END of its main BEGIN …​ END block is considered its terminator.

Examples of Sub-Procedures

  1. Subroutines in EXECUTE BLOCK

    EXECUTE BLOCK
  RETURNS (name VARCHAR(63))
AS
  -- Sub-procedure returning a list of tables
  DECLARE PROCEDURE get_tables
    RETURNS (table_name VARCHAR(63))
  AS
  BEGIN
    FOR SELECT RDB$RELATION_NAME
      FROM RDB$RELATIONS
      WHERE RDB$VIEW_BLR IS NULL
      INTO table_name
    DO SUSPEND;
  END
  -- Sub-procedure returning a list of views
  DECLARE PROCEDURE get_views
    RETURNS (view_name VARCHAR(63))
  AS
  BEGIN
    FOR SELECT RDB$RELATION_NAME
      FROM RDB$RELATIONS
      WHERE RDB$VIEW_BLR IS NOT NULL
      INTO view_name
    DO SUSPEND;
  END
BEGIN
  FOR SELECT table_name
    FROM get_tables
    UNION ALL
    SELECT view_name
    FROM get_views
    INTO name
  DO SUSPEND;
END

  2. With forward declaration and parameter with default value

    execute block returns (o integer)
as
    -- Forward declaration of P1.
    declare procedure p1(i integer = 1) returns (o integer);

    -- Forward declaration of P2.
    declare procedure p2(i integer) returns (o integer);

    -- Implementation of P1 should not re-declare parameter default value.
    declare procedure p1(i integer) returns (o integer)
    as
    begin
        execute procedure p2(i) returning_values o;
    end

    declare procedure p2(i integer) returns (o integer)
    as
    begin
        o = i;
    end
begin
    execute procedure p1 returning_values o;
    suspend;
end
See also

[fblangref50-psql-declfunc], CREATE PROCEDURE

BEGIN …​ END

Delimits a block of statements

Syntax
<block> ::=
  BEGIN
    [<compound_statement> ...]
  END

<compound_statement> ::= {<block> | <statement>}

The BEGIN …​ END construct is a two-part statement that wraps a block of statements that are executed as one unit of code.Each block starts with the keyword BEGIN and ends with the keyword END.Blocks can be nested a maximum depth of 512 nested blocks.A block can be empty, allowing them to act as stubs, without the need to write dummy statements.

The BEGIN …​ END itself should not be followed by a statement terminator (semicolon).However, when defining or altering a PSQL module in the isql utility, that application requires that the last END statement be followed by its own terminator character, that was previously switched — using SET TERM — to a string other than a semicolon.That terminator is not part of the PSQL syntax.

The final, or outermost, END statement in a trigger terminates the trigger.What the final END statement does in a stored procedure depends on the type of procedure:

  • In a selectable procedure, the final END statement returns control to the caller, returning SQLCODE 100, indicating that there are no more rows to retrieve

  • In an executable procedure, the final END statement returns control to the caller, along with the current values of any output parameters defined.

BEGIN …​ END Examples

A sample procedure from the employee.fdb database, showing simple usage of BEGIN …​ END blocks:
SET TERM ^;
CREATE OR ALTER PROCEDURE DEPT_BUDGET (
  DNO CHAR(3))
RETURNS (
  TOT DECIMAL(12,2))
AS
  DECLARE VARIABLE SUMB DECIMAL(12,2);
  DECLARE VARIABLE RDNO CHAR(3);
  DECLARE VARIABLE CNT  INTEGER;
BEGIN
  TOT = 0;

  SELECT BUDGET
  FROM DEPARTMENT
  WHERE DEPT_NO = :DNO
  INTO :TOT;

  SELECT COUNT(BUDGET)
  FROM DEPARTMENT
  WHERE HEAD_DEPT = :DNO
  INTO :CNT;

  IF (CNT = 0) THEN
    SUSPEND;

  FOR SELECT DEPT_NO
    FROM DEPARTMENT
    WHERE HEAD_DEPT = :DNO
    INTO :RDNO
  DO
  BEGIN
    EXECUTE PROCEDURE DEPT_BUDGET(:RDNO)
      RETURNING_VALUES :SUMB;
    TOT = TOT + SUMB;
  END

  SUSPEND;
END^
SET TERM ;^
See also

[fblangref50-psql-exit], SET TERM

IF …​ THEN …​ ELSE

Conditional branching

Syntax
IF (<condition>)
  THEN <compound_statement>
  [ELSE <compound_statement>]
Table 1. IF …​ THEN …​ ELSE Parameters
Argument Description

condition

A logical condition returning TRUE, FALSE or UNKNOWN

compound_statement

A single statement, or statements wrapped in BEGIN …​ END

The conditional branch statement IF …​ THEN is used to branch the execution process in a PSQL module.The condition is always enclosed in parentheses.If the condition returns the value TRUE, execution branches to the statement or the block of statements after the keyword THEN.If an ELSE is present, and the condition returns FALSE or UNKNOWN, execution branches to the statement or the block of statements after it.

Multi-Branch Decisions

PSQL does not provide more advanced multi-branch jumps, such as CASE or SWITCH.However, it is possible to chain IF …​ THEN …​ ELSE statements, see the example section below.Alternatively, the CASE statement from DSQL is available in PSQL and is able to satisfy at least some use cases in the manner of a switch:

CASE <test_expr>
  WHEN <expr> THEN <result>
  [WHEN <expr> THEN <result> ...]
  [ELSE <defaultresult>]
END

CASE
  WHEN <bool_expr> THEN <result>
  [WHEN <bool_expr> THEN <result> ...]
  [ELSE <defaultresult>]
END
Example in PSQL
...
C = CASE
      WHEN A=2 THEN 1
      WHEN A=1 THEN 3
      ELSE 0
    END;
...

IF Examples

  1. An example using the IF statement.Assume that the variables FIRST, LINE2 and LAST were declared earlier.

    ...
IF (FIRST IS NOT NULL) THEN
  LINE2 = FIRST || ' ' || LAST;
ELSE
  LINE2 = LAST;
...

  2. Given IF …​ THEN …​ ELSE is a statement, it is possible to chain them together.Assume that the INT_VALUE and STRING_VALUE variables were declared earlier.

    IF (INT_VALUE = 1) THEN
  STRING_VALUE = 'one';
ELSE IF (INT_VALUE = 2) THEN
  STRING_VALUE = 'two';
ELSE IF (INT_VALUE = 3) THEN
  STRING_VALUE = 'three';
ELSE
  STRING_VALUE = 'too much';

    This specific example can be replaced with a simple CASE or the DECODE function.

See also

[fblangref50-psql-while], CASE

WHILE …​ DO

Looping construct

Syntax
[label:]
WHILE (<condition>) DO
  <compound_statement>
Table 1. WHILE …​ DO Parameters
Argument Description

label

Optional label for LEAVE and CONTINUE.Follows the rules for identifiers.

condition

A logical condition returning TRUE, FALSE or UNKNOWN

compound_statement

A single statement, or statements wrapped in BEGIN …​ END

A WHILE statement implements the looping construct in PSQL.The statement or the block of statements will be executed as long as the condition returns TRUE.Loops can be nested to any depth.

WHILE …​ DO Examples

A procedure calculating the sum of numbers from 1 to I shows how the looping construct is used.

CREATE PROCEDURE SUM_INT (I INTEGER)
RETURNS (S INTEGER)
AS
BEGIN
  s = 0;
  WHILE (i > 0) DO
  BEGIN
    s = s + i;
    i = i - 1;
  END
END

Executing the procedure in isql:

EXECUTE PROCEDURE SUM_INT(4);

the result is:

S
==========
10
See also

[fblangref50-psql-ifthen], [fblangref50-psql-break], [fblangref50-psql-leave], [fblangref50-psql-continue], [fblangref50-psql-exit], [fblangref50-psql-forselect], [fblangref50-psql-forexec]

Trapping and Handling Errors

Firebird has a useful lexicon of PSQL statements and resources for trapping errors in modules and for handling them.Firebird uses built-in exceptions that are raised for errors occurring when working DML and DDL statements.

In PSQL code, exceptions are handled by means of the WHEN statement.Handling an exception in the code involves either fixing the problem in situ, or stepping past it;either solution allows execution to continue without returning an exception message to the client.

An exception results in execution being terminated in the current block.Instead of passing the execution to the END statement, the procedure moves outward through levels of nested blocks, starting from the block where the exception is caught, searching for the code of the handler that “knows” about this exception.It stops searching when it finds the first WHEN statement that can handle this exception.

System Exceptions

An exception is a message that is generated when an error occurs.

All exceptions handled by Firebird have predefined numeric values for context variables (symbols) and text messages associated with them.Error messages are output in English by default.Localized Firebird builds are available, where error messages are translated into other languages.

Complete listings of the system exceptions can be found in [fblangref50-appx02-errorcodes]:

Custom Exceptions

Custom exceptions can be declared in the database as persistent objects and called in PSQL code to signal specific errors;for example, to enforce certain business rules.A custom exception consists of an identifier, and a default message of 1021 bytes.For details, see CREATE EXCEPTION.

EXCEPTION

Throws a user-defined exception or rethrows an exception

Syntax
EXCEPTION [
    exception_name
    [ custom_message
    | USING (<value_list>)]
  ]

<value_list> ::= <val> [, <val> ...]
Table 1. EXCEPTION Statement Parameters
Argument Description

exception_name

Exception name

custom_message

Alternative message text to be returned to the caller interface when an exception is thrown.Maximum length of the text message is 1,021 bytes

val

Value expression that replaces parameter slots in the exception message text

The EXCEPTION statement with exception_name throws the user-defined exception with the specified name.An alternative message text of up to 1,021 bytes can optionally override the exception’s default message text.

The default exception message can contain slots for parameters that can be filled when throwing an exception.To pass parameter values to an exception, use the USING clause.Considering, in left-to-right order, each parameter passed in the exception-raising statement as “the Nth”, with N starting at 1:

  • If the Nth parameter is not passed, its slot is not replaced

  • If a NULL parameter is passed, the slot will be replaced with the string “*** null ***”

  • If more parameters are passed than are defined in the exception message, the surplus ones are ignored

  • The maximum number of parameters is 9

  • The maximum message length, including parameter values, is 1053 bytes

Note

The status vector is generated this code combination isc_except, <exception number>, isc_formatted_exception, <formatted exception message>, <exception parameters>.

The error code used (isc_formatted_exception) was introduced in Firebird 3.0, so the client must be at least version 3.0, or at least use the firebird.msg from version 3.0 or higher, to translate the status vector to a string.
Warning

If the message contains a parameter slot number that is greater than 9, the second and subsequent digits will be treated as literal text.For example @10 will be interpreted as slot 1 followed by a literal ‘0’.

As an example:

CREATE EXCEPTION ex1
  'something wrong in @1@2@3@4@5@6@7@8@9@10@11';
SET TERM ^;
EXECUTE BLOCK AS
BEGIN
  EXCEPTION ex1 USING ('a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i');
END^

This will produce the following output

Statement failed, SQLSTATE = HY000
exception 1
-EX1
-something wrong in abcdefghia0a1

Exceptions can be handled in a [fblangref50-psql-when] statement.If an exception is not handled in a module, then the effects of the actions executed inside this module are cancelled, and the caller program receives the exception (either the default text, or the custom text).

Within the exception-handling block — and only within it — the caught exception can be re-thrown by executing the EXCEPTION statement without parameters.If located outside the block, the re-thrown EXCEPTION call has no effect.

Custom exceptions are stored in the system table RDB$EXCEPTIONS.

EXCEPTION Examples

  1. Throwing an exception upon a condition in the SHIP_ORDER stored procedure:

    CREATE OR ALTER PROCEDURE SHIP_ORDER (
  PO_NUM CHAR(8))
AS
  DECLARE VARIABLE ord_stat  CHAR(7);
  DECLARE VARIABLE hold_stat CHAR(1);
  DECLARE VARIABLE cust_no   INTEGER;
  DECLARE VARIABLE any_po    CHAR(8);
BEGIN
  SELECT
    s.order_status,
    c.on_hold,
    c.cust_no
  FROM
    sales s, customer c
  WHERE
    po_number = :po_num AND
    s.cust_no = c.cust_no
  INTO :ord_stat,
       :hold_stat,
       :cust_no;

  IF (ord_stat = 'shipped') THEN
    EXCEPTION order_already_shipped;
  /* Other statements */
END

  2. Throwing an exception upon a condition and replacing the original message with an alternative message:

    CREATE OR ALTER PROCEDURE SHIP_ORDER (
  PO_NUM CHAR(8))
AS
  DECLARE VARIABLE ord_stat  CHAR(7);
  DECLARE VARIABLE hold_stat CHAR(1);
  DECLARE VARIABLE cust_no   INTEGER;
  DECLARE VARIABLE any_po    CHAR(8);
BEGIN
  SELECT
    s.order_status,
    c.on_hold,
    c.cust_no
  FROM
    sales s, customer c
  WHERE
    po_number = :po_num AND
    s.cust_no = c.cust_no
  INTO :ord_stat,
       :hold_stat,
       :cust_no;

  IF (ord_stat = 'shipped') THEN
    EXCEPTION order_already_shipped
      'Order status is "' || ord_stat || '"';
  /* Other statements */
END

  3. Using a parameterized exception:

    CREATE EXCEPTION EX_BAD_SP_NAME
  'Name of procedures must start with' '@ 1' ':' '@ 2' '' ;
...
CREATE TRIGGER TRG_SP_CREATE BEFORE CREATE PROCEDURE
AS
  DECLARE SP_NAME VARCHAR(255);
BEGIN
  SP_NAME = RDB$GET_CONTEXT ('DDL_TRIGGER' , 'OBJECT_NAME');
  IF (SP_NAME NOT STARTING 'SP_') THEN
    EXCEPTION EX_BAD_SP_NAME USING ('SP_', SP_NAME);
END

  4. Logging an error and re-throwing it in the WHEN block:

    CREATE PROCEDURE ADD_COUNTRY (
  ACountryName COUNTRYNAME,
  ACurrency VARCHAR(10))
AS
BEGIN
  INSERT INTO country (country,
                       currency)
  VALUES (:ACountryName,
          :ACurrency);
  WHEN ANY DO
  BEGIN
    -- write an error in log
    IN AUTONOMOUS TRANSACTION DO
      INSERT INTO ERROR_LOG (PSQL_MODULE,
                             GDS_CODE,
                             SQL_CODE,
                             SQL_STATE)
      VALUES ('ADD_COUNTRY',
              GDSCODE,
              SQLCODE,
              SQLSTATE);
    -- Re-throw exception
    EXCEPTION;
  END
END
See also

CREATE EXCEPTION, [fblangref50-psql-when]

WHEN …​ DO

Catches an exception for error handling

Syntax
WHEN {<error> [, <error> ...] | ANY}
DO <compound_statement>

<error> ::=
  { EXCEPTION exception_name
  | SQLCODE number
  | GDSCODE errcode
  | SQLSTATE sqlstate_code }
Table 1. WHEN …​ DO Statement Parameters
Argument Description

exception_name

Exception name

number

SQLCODE error code

errcode

Symbolic GDSCODE error name

sqlstate_code

String literal with the SQLSTATE error code

compound_statement

A single statement, or a block of statements

The WHEN …​ DO statement handles Firebird errors and user-defined exceptions.The statement catches all errors and user-defined exceptions listed after the keyword WHEN keyword.If WHEN is followed by the keyword ANY, the statement catches any error or user-defined exception, even if they have already been handled in a WHEN block located higher up.

The WHEN …​ DO statements must be located at the end of a block of statements, before the block’s END statement, and after any other statement.

The keyword DO is followed by a single statement, or statements wrapped in a BEGIN …​ END block, that handles the exception.The SQLCODE, GDSCODE, and SQLSTATE context variables are available in the context of this statement or block.Use the RDB$ERROR function to obtain the SQLCODE, GDSCODE, SQLSTATE, custom exception name and exception message.The EXCEPTION statement, without parameters, can also be used in this context to re-throw the error or exception.

Targeting GDSCODE

The argument for the WHEN GDSCODE clause is the symbolic name associated with the internally-defined exception, such as grant_obj_notfound for GDS error 335544551.

In a statement or block of statements of the DO clause, a GDSCODE context variable, containing the numeric code, becomes available.That numeric code is required if you want to compare a GDSCODE exception with a targeted error.To compare it with a specific error, you need to use a numeric values, for example 335544551 for grant_obj_notfound.

Similar context variables are available for SQLCODE and SQLSTATE.

The WHEN …​ DO statement or block is only executed when one of the events targeted by its conditions occurs at run-time.If the WHEN …​ DO statement is executed, even if it does nothing, execution will continue as if no error occurred: the error or user-defined exception neither terminates nor rolls back the operations of the trigger or stored procedure.

However, if the WHEN …​ DO statement or block does nothing to handle or resolve the error, the DML statement (SELECT, INSERT, UPDATE, DELETE, MERGE) that caused the error will be rolled back and none of the statements below it in the same block of statements are executed.

Important

  1. If the error is not caused by one of the DML statements (SELECT, INSERT, UPDATE, DELETE, MERGE), the entire block of statements will be rolled back, not only the one that caused an error.Any operations in the WHEN …​ DO statement will be rolled back as well.The same limitation applies to the EXECUTE PROCEDURE statement.Read an interesting discussion of the phenomenon in Firebird Tracker ticket firebird#4803.

  2. In selectable stored procedures, output rows that were already passed to the client in previous iterations of a FOR SELECT …​ DO …​ SUSPEND loop remain available to the client if an exception is thrown subsequently in the process of retrieving rows.

Scope of a WHEN …​ DO Statement

A WHEN …​ DO statement catches errors and exceptions in the current block of statements.It also catches exceptions from nested blocks, if those exceptions have not been handled in those blocks.

All changes made before the statement that caused the error are visible to a WHEN …​ DO statement.However, if you try to log them in an autonomous transaction, those changes are unavailable, because the transaction where the changes took place is not committed at the point when the autonomous transaction is started.Example 4, below, demonstrates this behaviour.

Tip

When handling exceptions, it is sometimes desirable to handle the exception by writing a log message to mark the fault and having execution continue past the faulty record.Logs can be written to regular tables, but there is a problem with that: the log records will “disappear” if an unhandled error causes the module to stop executing, and a rollback is performed.Use of external tables can be useful here, as data written to them is transaction-independent.The date inserted into a linked external file will still be there, regardless of whether the overall process succeeds or not.

Examples using WHEN…​DO

  1. Replacing the standard error with a custom one:

    CREATE EXCEPTION COUNTRY_EXIST '';
SET TERM ^;
CREATE PROCEDURE ADD_COUNTRY (
  ACountryName COUNTRYNAME,
  ACurrency VARCHAR(10) )
AS
BEGIN
  INSERT INTO country (country, currency)
    VALUES (:ACountryName, :ACurrency);

  WHEN SQLCODE -803 DO
    EXCEPTION COUNTRY_EXIST 'Country already exists!';
END^
SET TERM ^;

  2. Logging an error and re-throwing it in the WHEN block:

    CREATE PROCEDURE ADD_COUNTRY (
  ACountryName COUNTRYNAME,
  ACurrency VARCHAR(10) )
AS
BEGIN
  INSERT INTO country (country,
                       currency)
  VALUES (:ACountryName,
          :ACurrency);
  WHEN ANY DO
  BEGIN
    -- write an error in log
    IN AUTONOMOUS TRANSACTION DO
      INSERT INTO ERROR_LOG (PSQL_MODULE,
                             GDS_CODE,
                             SQL_CODE,
                             SQL_STATE,
                             MESSAGE)
      VALUES ('ADD_COUNTRY',
              GDSCODE,
              SQLCODE,
              SQLSTATE,
              RDB$ERROR(MESSAGE));
    -- Re-throw exception
    EXCEPTION;
  END
END

  3. Handling several errors in one WHEN block

    ...
WHEN GDSCODE GRANT_OBJ_NOTFOUND,
	 GDSCODE GRANT_FLD_NOTFOUND,
	 GDSCODE GRANT_NOPRIV,
	 GDSCODE GRANT_NOPRIV_ON_BASE
DO
BEGIN
  EXECUTE PROCEDURE LOG_GRANT_ERROR(GDSCODE,
    RDB$ERROR(MESSAGE);
  EXIT;
END
...

  4. Catching errors using the SQLSTATE code

    EXECUTE BLOCK
AS
  DECLARE VARIABLE I INT;
BEGIN
  BEGIN
    I = 1/0;
    WHEN SQLSTATE '22003' DO
      EXCEPTION E_CUSTOM_EXCEPTION
        'Numeric value out of range.';
    WHEN SQLSTATE '22012' DO
      EXCEPTION E_CUSTOM_EXCEPTION
        'Division by zero.';
    WHEN SQLSTATE '23000' DO
      EXCEPTION E_CUSTOM_EXCEPTION
       'Integrity constraint violation.';
  END
END
See also

[fblangref50-psql-exception], CREATE EXCEPTION, SQLCODE and GDSCODE Error Codes and Message Texts and SQLSTATE Codes and Message Texts, GDSCODE, SQLCODE, SQLSTATE, RDB$ERROR()