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NULL in Unique Keys

Firebird’s SQL-compliant rules for UNIQUE constraints allow one or more NULLs in a column with a UNIQUE constraint.This makes it possible to define a UNIQUE constraint on a column that does not have the NOT NULL constraint.

For UNIQUE keys that span multiple columns, the logic is a little complicated:

  • Multiple rows having null in all the columns of the key are allowed

  • Multiple rows having keys with different combinations of nulls and non-null values are allowed

  • Multiple rows having the same key columns null and the rest filled with non-null values are allowed, provided the non-null values differ in at least one column

  • Multiple rows having the same key columns null and the rest filled with non-null values that are the same in every column will violate the constraint

The rules for uniqueness can be summarised thus:

In principle, all nulls are considered distinct. However, if two rows have exactly the same key columns filled with non-null values, the NULL columns are ignored and the uniqueness is determined on the non-null columns as though they constituted the entire key.
Illustration
RECREATE TABLE t( x int, y int, z int, unique(x,y,z));
INSERT INTO t values( NULL, 1, 1 );
INSERT INTO t values( NULL, NULL, 1 );
INSERT INTO t values( NULL, NULL, NULL );
INSERT INTO t values( NULL, NULL, NULL ); -- Permitted
INSERT INTO t values( NULL, NULL, 1 );    -- Not permitted

FOREIGN KEY

A foreign key ensures that the participating column(s) can contain only values that also exist in the referenced column(s) in the master table.These referenced columns are often called target columns.They must be the primary key or a unique key in the target table.They need not have a NOT NULL constraint defined on them although, if they are the primary key, they will, of course, have that constraint.

The foreign key columns in the referencing table itself do not require a NOT NULL constraint.

A single-column foreign key can be defined in the column declaration, using the keyword REFERENCES:

... ,
  ARTIFACT_ID INTEGER REFERENCES COLLECTION (ARTIFACT_ID),

The column ARTIFACT_ID in the example references a column of the same name in the table COLLECTIONS.

Both single-column and multi-column foreign keys can be defined at the table level.For a multi-column foreign key, the table-level declaration is the only option.

...
  CONSTRAINT FK_ARTSOURCE FOREIGN KEY(DEALER_ID, COUNTRY)
    REFERENCES DEALER (DEALER_ID, COUNTRY),

Notice that the column names in the referenced (“master”) table may differ from those in the foreign key.

Note

If no target columns are specified, the foreign key automatically references the target table’s primary key.

docnext count = 32

Foreign Key Actions

With the sub-clauses ON UPDATE and ON DELETE it is possible to specify an action to be taken on the affected foreign key column(s) when referenced values in the master table are changed:

NO ACTION

(the default) — Nothing is done

CASCADE

The change in the master table is propagated to the corresponding row(s) in the child table.If a key value changes, the corresponding key in the child records changes to the new value;if the master row is deleted, the child records are deleted.

SET DEFAULT

The foreign key columns in the affected rows will be set to their default values as they were when the foreign key constraint was defined.

SET NULL

The foreign key columns in the affected rows will be set to NULL.

The specified action, or the default NO ACTION, could cause a foreign key column to become invalid.For example, it could get a value that is not present in the master table.Such condition will cause the operation on the master table to fail with an error message.

Example
...
  CONSTRAINT FK_ORDERS_CUST
    FOREIGN KEY (CUSTOMER) REFERENCES CUSTOMERS (ID)
      ON UPDATE CASCADE ON DELETE SET NULL

CHECK Constraint

The CHECK constraint defines the condition the values inserted in this column or row must satisfy.A condition is a logical expression (also called a predicate) that can return the TRUE, FALSE and UNKNOWN values.A condition is considered satisfied if the predicate returns TRUE or value UNKNOWN (equivalent to NULL).If the predicate returns FALSE, the value will not be accepted.This condition is used for inserting a new row into the table (the INSERT statement) and for updating the existing value of the table column (the UPDATE statement) and also for statements where one of these actions may take place (UPDATE OR INSERT, MERGE).

Important

A CHECK constraint on a domain-based column does not replace an existing CHECK condition on the domain, but becomes an addition to it.The Firebird engine has no way, during definition, to verify that the extra CHECK does not conflict with the existing one.

CHECK constraints — whether defined at table level or column level — refer to table columns by their names.The use of the keyword VALUE as a placeholder — as in domain CHECK constraints — is not valid in the context of defining constraints in a table.

Example

with two column-level constraints and one at table-level:

CREATE TABLE PLACES (
  ...
  LAT DECIMAL(9, 6) CHECK (ABS(LAT) <=  90),
  LON DECIMAL(9, 6) CHECK (ABS(LON) <= 180),
  ...
  CONSTRAINT CHK_POLES CHECK (ABS(LAT) < 90 OR LON = 0)
);

NOT NULL Constraint

In Firebird, columns are nullable by default.The NOT NULL constraint specifies that the column cannot take NULL in place of a value.

A NOT NULL constraint can only be defined as a column constraint, not as a table constraint.

SQL SECURITY Clause

The SQL SECURITY clause specifies the security context for executing functions referenced in computed columns, and check constraints, and the default context used for triggers fired for this table.When SQL Security is not specified, the default value of the database is applied at runtime.

See also SQL Security in chapter Security.

Replication Management

When the database has been configured using ALTER DATABASE INCLUDE ALL TO PUBLICATION, new tables will automatically be added for publication, unless overridden using the DISABLE PUBLICATION clause.

If the database has not been configured for INCLUDE ALL (or has later been reconfigured using ALTER DATABASE EXCLUDE ALL FROM PUBLICATION), new tables will not automatically be added for publication.To include tables for publication, the ENABLE PUBLICATION clause must be used.

ALTER TABLE

Alters a table

Available in

DSQL, ESQL

Syntax
ALTER TABLE tablename
  <operation> [, <operation> ...]

<operation> ::=
    ADD <col_def>
  | ADD <tconstraint>
  | DROP colname
  | DROP CONSTRAINT constr_name
  | ALTER [COLUMN] colname <col_mod>
  | ALTER SQL SECURITY {INVOKER | DEFINER}
  | DROP SQL SECURITY
  | {ENABLE | DISABLE} PUBLICATION

<col_mod> ::=
    TO newname
  | POSITION newpos
  | <regular_col_mod>
  | <computed_col_mod>
  | <identity_col_mod>

<regular_col_mod> ::=
    TYPE {<datatype> | domainname}
  | SET DEFAULT {<literal> | NULL | <context_var>}
  | DROP DEFAULT
  | {SET | DROP} NOT NULL

<computed_col_mod> ::=
    [TYPE <datatype>] {COMPUTED [BY] | GENERATED ALWAYS AS} (<expression>)

<identity_col_mod> ::=
    SET GENERATED {ALWAYS | BY DEFAULT} [<identity_mod_option>...]
  | <identity_mod_options>...
  | DROP IDENTITY

<identity_mod_options> ::=
    RESTART [WITH restart_value]
  | SET INCREMENT [BY] inc_value

!! See CREATE TABLE syntax for further rules !!
Table 1. ALTER TABLE Statement Parameters
Parameter Description

tablename

Name (identifier) of the table

operation

One of the available operations altering the structure of the table

colname

Name (identifier) for a column in the table.The maximum length is 63 characters.Must be unique in the table.

domain_name

Domain name

newname

New name (identifier) for the column.The maximum length is 63 characters.Must be unique in the table.

newpos

The new column position (an integer between 1 and the number of columns in the table)

other_table

The name of the table referenced by the foreign key constraint

literal

A literal value that is allowed in the given context

context_var

A context variable whose type is allowed in the given context

check_condition

The condition of a CHECK constraint that will be satisfied if it evaluates to TRUE or UNKNOWN/NULL

restart_value

The first value of the identity column after restart

inc_value

The increment (or step) value of the identity column;zero (0) is not allowed.

The ALTER TABLE statement changes the structure of an existing table.With one ALTER TABLE statement it is possible to perform multiple operations, adding/dropping columns and constraints and also altering column specifications.

Multiple operations in an ALTER TABLE statement are separated with commas.

Version Count Increments

Some changes in the structure of a table increment the metadata change counter (“version count”) assigned to every table.The number of metadata changes is limited to 255 for each table, or 32,000 for each view.Once the counter reaches this limit, you will not be able to make any further changes to the structure of the table or view without resetting the counter.

To reset the metadata change counter
You need to back up and restore the database using the gbak utility.

The SET NOT NULL and DROP NOT NULL Clauses

The SET NOT NULL clause adds a NOT NULL constraint on an existing table column.Contrary to definition in CREATE TABLE, it is not possible to specify a constraint name.

Note

The successful addition of the NOT NULL constraint is subject to a full data validation on the table, so ensure that the column has no nulls before attempting the change.

An explicit NOT NULL constraint on domain-based column overrides domain settings.In this scenario, changing the domain to be nullable does not extend to a table column.

Dropping the NOT NULL constraint from the column if its type is a domain that also has a NOT NULL constraint, has no observable effect until the NOT NULL constraint is dropped from the domain as well.

The COMPUTED [BY] or GENERATED ALWAYS AS Clauses

The data type and expression underlying a computed column can be modified using a COMPUTED [BY] or GENERATED ALWAYS AS clause in the ALTER TABLE ALTER [COLUMN] statement.Conversion of a regular column to a computed one and vice versa is not permitted.

Changing Identity Columns

For identity columns (SET GENERATED {ALWAYS | BY DEFAULT}) it is possible to modify several properties using the following clauses.

Identity Type

The SET GENERATED {ALWAYS | BY DEFAULT} changes an identity column from ALWAYS to BY DEFAULT and vice versa.It is not possible to use this to change a regular column to an identity column.

RESTART

The RESTART clause restarts the sequence used for generating identity values.If only the RESTART clause is specified, then the sequence resets to the initial value specified when the identity column was defined.If the optional WITH restart_value clause is specified, the sequence will restart with the specified value.

Note

In Firebird 3.0, RESTART WITH restart_value would also change the configured initial value to restart_value.This was not compliant with the SQL standard, so since Firebird 4.0, RESTART WITH restart_value will only restart the sequence with the specified value.Subsequent RESTARTs (without WITH) will use the START WITH value specified when the identity column was defined.

It is currently not possible to change the configured start value.

SET INCREMENT

The SET INCREMENT clause changes the increment of the identity column.

DROP IDENTITY

The DROP IDENTITY clause will change an identity column to a regular column.

Note

It is not possible to change a regular column to an identity column.

Changing SQL Security

Using the ALTER SQL SECURITY or DROP SQL SECURITY clauses, it is possible to change or drop the SQL Security property of a table.After dropping SQL Security, the default value of the database is applied at runtime.

Note

If the SQL Security property is changed for a table, triggers that do not have an explicit SQL Security property will not see the effect of the change until the next time the trigger is loaded into the metadata cache.

Replication Management

To stop replicating a table, use the DISABLE PUBLICATION clause.To start replicating a table, use the ENABLE PUBLICATION clause.

The change in publication status takes effect at commit.

Attributes that Cannot Be Altered

The following alterations are not supported:

  • Changing the collation of a character type column

Who Can Alter a Table?

The ALTER TABLE statement can be executed by:

  • Administrators

  • The owner of the table

  • Users with the ALTER ANY TABLE privilege

Examples Using ALTER TABLE

  1. Adding the CAPITAL column to the COUNTRY table.

    ALTER TABLE COUNTRY
  ADD CAPITAL VARCHAR(25);

  2. Adding the CAPITAL column with the NOT NULL and UNIQUE constraint and deleting the CURRENCY column.

    ALTER TABLE COUNTRY
  ADD CAPITAL VARCHAR(25) NOT NULL UNIQUE,
  DROP CURRENCY;

  3. Adding the CHK_SALARY check constraint and a foreign key to the JOB table.

    ALTER TABLE JOB
  ADD CONSTRAINT CHK_SALARY CHECK (MIN_SALARY < MAX_SALARY),
  ADD FOREIGN KEY (JOB_COUNTRY) REFERENCES COUNTRY (COUNTRY);

  4. Setting default value for the MODEL field, changing the type of the ITEMID column and renaming the MODELNAME column.

    ALTER TABLE STOCK
  ALTER COLUMN MODEL SET DEFAULT 1,
  ALTER COLUMN ITEMID TYPE BIGINT,
  ALTER COLUMN MODELNAME TO NAME;

  5. Restarting the sequence of an identity column.

    ALTER TABLE objects
  ALTER ID RESTART WITH 100;

  6. Changing the computed columns NEW_SALARY and SALARY_CHANGE.

    ALTER TABLE SALARY_HISTORY
  ALTER NEW_SALARY GENERATED ALWAYS AS
    (OLD_SALARY + OLD_SALARY * PERCENT_CHANGE / 100),
  ALTER SALARY_CHANGE COMPUTED BY
    (OLD_SALARY * PERCENT_CHANGE / 100);
See also

[fblangref50-ddl-tbl-create], [fblangref50-ddl-tbl-drop], [fblangref50-ddl-domn-create]

The ADD Clause

With the ADD clause you can add a new column or a new table constraint.The syntax for defining the column and the syntax of defining the table constraint correspond with those described for [fblangref50-ddl-tbl-create] statement.

Effect on Version Count

  • Each time a new column is added, the metadata change counter is increased by one

  • Adding a new table constraint does not increase the metadata change counter

Caution
Points to Be Aware of

  1. Adding a column with a NOT NULL constraint without a DEFAULT value will fail if the table has existing rows.When adding a non-nullable column, it is recommended either to set a default value for it, or to create it as nullable, update the column in existing rows with a non-null value, and then add a NOT NULL constraint.

  2. When a new CHECK constraint is added, existing data is not tested for compliance.Prior testing of existing data against the new CHECK expression is recommended.

  3. Although adding an identity column is supported, this will only succeed if the table is empty.Adding an identity column will fail if the table has one or more rows.

The DROP Clause

The DROP colname clause deletes the specified column from the table.An attempt to drop a column will fail if anything references it.Consider the following items as sources of potential dependencies:

  • column or table constraints

  • indexes

  • stored procedures, functions and triggers

  • views

Effect on Version Count

  • Each time a column is dropped, the table’s metadata change counter is increased by one.

The DROP CONSTRAINT Clause

The DROP CONSTRAINT clause deletes the specified column-level or table-level constraint.

A PRIMARY KEY or UNIQUE key constraint cannot be deleted if it is referenced by a FOREIGN KEY constraint in another table.It will be necessary to drop that FOREIGN KEY constraint before attempting to drop the PRIMARY KEY or UNIQUE key constraint it references.

Effect on Version Count

  • Deleting a column constraint or a table constraint does not increase the metadata change counter.

The ALTER [COLUMN] Clause

With the ALTER [COLUMN] clause, attributes of existing columns can be modified without the need to drop and re-add the column.Permitted modifications are:

  • change the name (does not affect the metadata change counter)

  • change the data type (increases the metadata change counter by one)

  • change the column position in the column list of the table (does not affect the metadata change counter)

  • delete the default column value (does not affect the metadata change counter)

  • set a default column value or change the existing default (does not affect the metadata change counter)

  • change the type and expression for a computed column (does not affect the metadata change counter)

  • set the NOT NULL constraint (does not affect the metadata change counter)

  • drop the NOT NULL constraint (does not affect the metadata change counter)

  • change the type of an identity column, or change an identity column to a regular column

  • restart an identity column

  • change the increment of an identity column

Renaming a Column: the TO Clause

The TO keyword with a new identifier renames an existing column.The table must not have an existing column that has the same identifier.

It will not be possible to change the name of a column that is included in any constraint: primary key, unique key, foreign key, or CHECK constraints of the table.

Renaming a column will also be disallowed if the column is used in any stored PSQL module or view.

Changing the Data Type of a Column: the TYPE Clause

The keyword TYPE changes the data type of an existing column to another, allowable type.A type change that might result in data loss will be disallowed.As an example, the number of characters in the new type for a CHAR or VARCHAR column cannot be smaller than the existing specification for it.

If the column was declared as an array, no change to its type or its number of dimensions is permitted.

The data type of a column that is involved in a foreign key, primary key or unique constraint cannot be changed at all.

Changing the Position of a Column: the POSITION Clause

The POSITION keyword changes the position of an existing column in the notional “left-to-right” layout of the record.

Numbering of column positions starts at 1.

  • If a position less than 1 is specified, an error message will be returned

  • If a position number is greater than the number of columns in the table, its new position will be adjusted silently to match the number of columns.

The DROP DEFAULT and SET DEFAULT Clauses

The optional DROP DEFAULT clause deletes the current default value for the column.

  • If the column is based on a domain with a default value, the default value will revert to the domain default

  • An error will be raised if an attempt is made to delete the default value of a column which has no default value or whose default value is domain-based

The optional SET DEFAULT clause sets a default value for the column.If the column already has a default value, it will be replaced with the new one.The default value applied to a column always overrides one inherited from a domain.

DROP TABLE

Drops a table

Available in

DSQL, ESQL

Syntax
DROP TABLE tablename
Table 1. DROP TABLE Statement Parameter
Parameter Description

tablename

Name (identifier) of the table

The DROP TABLE statement drops (deletes) an existing table.If the table has dependencies, the DROP TABLE statement will fail with an error.

When a table is dropped, all its triggers and indexes will be deleted as well.

Who Can Drop a Table?

The DROP TABLE statement can be executed by:

  • Administrators

  • The owner of the table

  • Users with the DROP ANY TABLE privilege

Example of DROP TABLE

Dropping the COUNTRY table.
DROP TABLE COUNTRY;
See also

[fblangref50-ddl-tbl-create], [fblangref50-ddl-tbl-alter], [fblangref50-ddl-tbl-recreate]

RECREATE TABLE

Drops a table if it exists, and creates a table

Available in

DSQL

Syntax
RECREATE [GLOBAL TEMPORARY] TABLE tablename
  [EXTERNAL [FILE] 'filespec']
  (<col_def> [, {<col_def> | <tconstraint>} ...])
  [{<table_attrs> | <gtt_table_attrs>}]

See the CREATE TABLE section for the full syntax of CREATE TABLE and descriptions of defining tables, columns and constraints.

RECREATE TABLE creates or recreates a table.If a table with this name already exists, the RECREATE TABLE statement will try to drop it and create a new one.Existing dependencies will prevent the statement from executing.

Example of RECREATE TABLE

Creating or recreating the COUNTRY table.
RECREATE TABLE COUNTRY (
  COUNTRY COUNTRYNAME NOT NULL PRIMARY KEY,
  CURRENCY VARCHAR(10) NOT NULL
);
See also

[fblangref50-ddl-tbl-create], [fblangref50-ddl-tbl-drop]