The Firebird 5.0 Language Reference | CREATE VIEW

Updatable Views

A view can be updatable or read-only.If a view is updatable, the data retrieved when this view is called can be changed by the DML statements INSERT, UPDATE, DELETE, UPDATE OR INSERT or MERGE.Changes made in an updatable view are applied to the underlying table(s).

A read-only view can be made updatable with the use of triggers.Once triggers have been defined on a view, changes posted to it will never be written automatically to the underlying table, even if the view was updatable to begin with.It is the responsibility of the programmer to ensure that the triggers update (or delete from, or insert into) the base tables as needed.

A view will be automatically updatable if all the following conditions are met:

the SELECT statement queries only one table or one updatable view
the SELECT statement does not call any stored procedures
each base table (or base view) column not present in the view definition meets one of the following conditions:
- it is nullable
- it has a non-NULL default value
- it has a trigger that supplies a permitted value
the SELECT statement contains no fields derived from subqueries or other expressions
the SELECT statement does not contain fields defined through aggregate functions (MIN, MAX, AVG, SUM, COUNT, LIST, etc.), statistical functions (CORR, COVAR_POP, COVAR_SAMP, etc.), linear regression functions (REGR_AVGX, REGR_AVGY, etc.) or any type of window function
the SELECT statement contains no ORDER BY, GROUP BY or HAVING clause
the SELECT statement does not include the keyword DISTINCT or row-restrictive keywords such as ROWS, FIRST, SKIP, OFFSET or FETCH

Note

The RETURNING clause and updatable views

The RETURNING clause of a DML statement used on a view made updatable using triggers may not always report the correct values.For example, values of identity column, computed columns, default values, or other expressions performed by the trigger will not be automatically reflected in the RETURNING columns.

To report the right values in RETURNING, the trigger will need to explicitly assign those values to the columns of the NEW record.

`WITH CHECK OPTION`

The optional WITH CHECK OPTION clause requires an updatable view to check whether new or updated data meet the condition specified in the WHERE clause of the SELECT statement.Every attempt to insert a new record or to update an existing one is checked whether the new or updated record would meet the WHERE criteria.If they fail the check, the operation is not performed and an error is raised.

WITH CHECK OPTION can be specified only in a CREATE VIEW statement in which a WHERE clause is present to restrict the output of the main SELECT statement.An error message is returned otherwise.

Important

Please note:

If WITH CHECK OPTION is used, the engine checks the input against the WHERE clause before passing anything to the base relation.Therefore, if the check on the input fails, any default clauses or triggers on the base relation that might have been designed to correct the input will never come into action.

Furthermore, view fields omitted from the INSERT statement are passed as NULLs to the base relation, regardless of their presence or absence in the WHERE clause.As a result, base table defaults defined on such fields will not be applied.Triggers, on the other hand, will fire and work as expected.

For views that do not have WITH CHECK OPTION, fields omitted from the INSERT statement are not passed to the base relation at all, so any defaults will be applied.

Who Can Create a View?

The CREATE VIEW statement can be executed by:

Administrators
Users with the CREATE VIEW privilege

The creator of a view becomes its owner.

To create a view, a non-admin user also needs at least SELECT access to the underlying table(s) and/or view(s), and the EXECUTE privilege on any selectable stored procedures involved.

To enable insertions, updates and deletions through the view, the creator/owner must also possess the corresponding INSERT, UPDATE and DELETE rights on the underlying object(s).

Granting other users privileges on the view is only possible if the view owner has these privileges on the underlying objects WITH GRANT OPTION.This will always be the case if the view owner is also the owner of the underlying objects.

Examples of Creating Views

Creating view returning the JOB_CODE and JOB_TITLE columns only for those jobs where MAX_SALARY is less than $15,000.
```
CREATE VIEW ENTRY_LEVEL_JOBS AS
SELECT JOB_CODE, JOB_TITLE
FROM JOB
WHERE MAX_SALARY < 15000;
```
Creating a view returning the JOB_CODE and JOB_TITLE columns only for those jobs where MAX_SALARY is less than $15,000.Whenever a new record is inserted or an existing record is updated, the MAX_SALARY < 15000 condition will be checked.If the condition is not true, the insert/update operation will be rejected.
```
CREATE VIEW ENTRY_LEVEL_JOBS AS
SELECT JOB_CODE, JOB_TITLE
FROM JOB
WHERE MAX_SALARY < 15000
WITH CHECK OPTION;
```

Creating a view with an explicit column list.

CREATE VIEW PRICE_WITH_MARKUP (
  CODE_PRICE,
  COST,
  COST_WITH_MARKUP
) AS
SELECT
  CODE_PRICE,
  COST,
  COST * 1.1
FROM PRICE;

Creating a view with the help of aliases for fields in the SELECT statement (the same result as in Example 3).
```
CREATE VIEW PRICE_WITH_MARKUP AS
SELECT
  CODE_PRICE,
  COST,
  COST * 1.1 AS COST_WITH_MARKUP
FROM PRICE;
```

Creating a read-only view based on two tables and a stored procedure.

CREATE VIEW GOODS_PRICE AS
SELECT
  goods.name AS goodsname,
  price.cost AS cost,
  b.quantity AS quantity
FROM
  goods
  JOIN price ON goods.code_goods = price.code_goods
  LEFT JOIN sp_get_balance(goods.code_goods) b ON 1 = 1;

`ALTER VIEW`

Alters a view

Available in

DSQL

Syntax

ALTER VIEW viewname [<full_column_list>]
    AS <select_statement>
    [WITH CHECK OPTION]

<full_column_list> ::= (colname [, colname ...])

Table 1. `ALTER VIEW` Statement Parameters
Parameter	Description
viewname	Name of an existing view
select_statement	SELECT statement
full_column_list	The list of columns in the view
colname	View column name.Duplicate column names are not allowed.

Use the ALTER VIEW statement for changing the definition of an existing view.Privileges for views remain intact and dependencies are not affected.

The syntax of the ALTER VIEW statement corresponds with that of CREATE VIEW.

Caution

Be careful when you change the number of columns in a view.Existing application code and PSQL modules that access the view may become invalid.For information on how to detect this kind of problem in stored procedures and trigger, see The RDB$VALID_BLR Field in the Appendix.

Who Can Alter a View?

The ALTER VIEW statement can be executed by:

Administrators
The owner of the view
Users with the ALTER ANY VIEW privilege

Example using `ALTER VIEW`

Altering the view PRICE_WITH_MARKUP

ALTER VIEW PRICE_WITH_MARKUP (
  CODE_PRICE,
  COST,
  COST_WITH_MARKUP
) AS
SELECT
  CODE_PRICE,
  COST,
  COST * 1.15
FROM PRICE;

`CREATE OR ALTER VIEW`

Creates a view if it doesn’t exist, or alters a view

Available in

DSQL

Syntax

CREATE OR ALTER VIEW viewname [<full_column_list>]
  AS <select_statement>
  [WITH CHECK OPTION]

<full_column_list> ::= (colname [, colname ...])

Table 1. `CREATE OR ALTER VIEW` Statement Parameters
Parameter	Description
viewname	Name of a view which may or may not exist
select_statement	SELECT statement
full_column_list	The list of columns in the view
colname	View column name.Duplicate column names are not allowed.

Use the CREATE OR ALTER VIEW statement for changing the definition of an existing view or creating it if it does not exist.Privileges for an existing view remain intact and dependencies are not affected.

The syntax of the CREATE OR ALTER VIEW statement corresponds with that of CREATE VIEW.

Example of `CREATE OR ALTER VIEW`

Creating the new view PRICE_WITH_MARKUP view or altering it if it already exists

CREATE OR ALTER VIEW PRICE_WITH_MARKUP (
  CODE_PRICE,
  COST,
  COST_WITH_MARKUP
) AS
SELECT
  CODE_PRICE,
  COST,
  COST * 1.15
FROM PRICE;

`DROP VIEW`

Drops a view

Available in

DSQL

Syntax

DROP VIEW viewname

Table 1. `DROP VIEW` Statement Parameter
Parameter	Description
viewname	View name

The DROP VIEW statement drops (deletes) an existing view.The statement will fail if the view has dependencies.

Who Can Drop a View?

The DROP VIEW statement can be executed by:

Administrators
The owner of the view
Users with the DROP ANY VIEW privilege

Example

Deleting the PRICE_WITH_MARKUP view

DROP VIEW PRICE_WITH_MARKUP;

`RECREATE VIEW`

Drops a view if it exists, and creates a view

Available in

DSQL

Syntax

RECREATE VIEW viewname [<full_column_list>]
  AS <select_statement>
  [WITH CHECK OPTION]

<full_column_list> ::= (colname [, colname ...])

Table 1. `RECREATE VIEW` Statement Parameters
Parameter	Description
viewname	View name.The maximum length is 63 characters
select_statement	SELECT statement
full_column_list	The list of columns in the view
colname	View column name.Duplicate column names are not allowed.

Creates or recreates a view.If there is a view with this name already, the engine will try to drop it before creating the new instance.If the existing view cannot be dropped, because of dependencies or insufficient rights, for example, RECREATE VIEW fails with an error.

Example of `RECREATE VIEW`

Creating the new view PRICE_WITH_MARKUP view or recreating it, if it already exists

RECREATE VIEW PRICE_WITH_MARKUP (
  CODE_PRICE,
  COST,
  COST_WITH_MARKUP
) AS
SELECT
  CODE_PRICE,
  COST,
  COST * 1.15
FROM PRICE;

Updatable Views

Updatable Views

WITH CHECK OPTION

Who Can Create a View?

Examples of Creating Views

ALTER VIEW

Who Can Alter a View?

Example using ALTER VIEW

CREATE OR ALTER VIEW

Example of CREATE OR ALTER VIEW

DROP VIEW

Who Can Drop a View?

Example

RECREATE VIEW

Example of RECREATE VIEW

`WITH CHECK OPTION`

`ALTER VIEW`

Example using `ALTER VIEW`

`CREATE OR ALTER VIEW`

Example of `CREATE OR ALTER VIEW`

`DROP VIEW`

`RECREATE VIEW`

Example of `RECREATE VIEW`