5.6 VIEW

A view is a virtual table that is actually a stored and named SELECT query for retrieving data of any complexity. Data can be retrieved from one or more tables, from other views and also from selectable stored procedures.

Unlike regular tables in relational databases, a view is not an independent data set stored in the database. The result is dynamically created as a data set when the view is selected.

The metadata of a view are available to the process that generates the binary code for stored procedures and triggers, just as though they were concrete tables storing persistent data.

5.6.1 CREATE VIEW

Used forCreating a view

Available inDSQL

Syntax

CREATE VIEW viewname [<full_column_list>]
  AS <select_statement>
  [WITH CHECK OPTION]
 
<full_column_list> ::= (colname [, colname ...])

Table 5.6.1.1 CREATE VIEW Statement Parameters

The CREATE VIEW statement creates a new view. The identifier (name) of a view must be unique among the names of all views, tables and stored procedures in the database.

The name of the new view can be followed by the list of column names that should be returned to the caller when the view is invoked. Names in the list do not have to be related to the names of the columns in the base tables from which they derive.

If the view column list is omitted, the system will use the column names and/or aliases from the SELECT statement. If duplicate names or non-aliased expression-derived columns make it impossible to obtain a valid list, creation of the view fails with an error.

The number of columns in the view’s list must exactly match the number of columns in the selection list of the underlying SELECT statement in the view definition.

Additional Points

• If the full list of columns is specified, it makes no sense to specify aliases in the SELECT statement because the names in the column list will override them

• The column list is optional if all the columns in the SELECT are explicitly named and are unique in the selection list

5.6.1.1 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 updateable 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 updateable 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

5.6.1.2 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 appropriate error message is returned.

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.

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.

5.6.1.3 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. It will always be the case if the view owner is also the owner of the underlying objects.

5.6.1.4 Examples of Creating Views

1. 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;

2. 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;

3. 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;

4. 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;

5. 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;

See alsoSection 5.6.2, ALTER VIEW, Section 5.6.3, CREATE OR ALTER VIEW, Section 5.6.5, RECREATE VIEW, Section 5.6.4, DROP VIEW

5.6.2 ALTER VIEW

Used forModifying an existing view

Available inDSQL

Syntax

ALTER VIEW viewname [<full_column_list>]
    AS <select_statement>
    [WITH CHECK OPTION]
 
<full_column_list> ::= (colname [, colname ...])

Table 5.6.2.1 ALTER VIEW Statement Parameters

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 completely 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.

5.6.2.1 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

5.6.2.2 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;

See alsoSection 5.6.1, CREATE VIEW, Section 5.6.3, CREATE OR ALTER VIEW, Section 5.6.5, RECREATE VIEW

5.6.3 CREATE OR ALTER VIEW

Used forCreating a new view or altering an existing view.

Available inDSQL

Syntax

CREATE OR ALTER VIEW viewname [<full_column_list>]
  AS <select_statement>
  [WITH CHECK OPTION]
 
<full_column_list> ::= (colname [, colname ...])

Table 5.6.3.1 CREATE OR ALTER VIEW Statement Parameters

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 completely with that of CREATE VIEW.

5.6.3.1 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;

See alsoSection 5.6.1, CREATE VIEW, Section 5.6.2, ALTER VIEW, Section 5.6.5, RECREATE VIEW

5.6.4 DROP VIEW

Used forDeleting (dropping) a view

Available inDSQL

Syntax

DROP VIEW viewname

Table 5.6.4.1 DROP VIEW Statement Parameter

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

5.6.4.1 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

5.6.4.2 Example

Deleting the PRICE_WITH_MARKUP view

DROP VIEW PRICE_WITH_MARKUP;

See alsoSection 5.6.1, CREATE VIEW, Section 5.6.5, RECREATE VIEW, Section 5.6.3, CREATE OR ALTER VIEW

5.6.5 RECREATE VIEW

Used forCreating a new view or recreating an existing view

Available inDSQL

Syntax

RECREATE VIEW viewname [<full_column_list>]
  AS <select_statement>
  [WITH CHECK OPTION]
 
<full_column_list> ::= (colname [, colname ...])

Table 5.6.5.1 RECREATE VIEW Statement Parameters

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.

5.6.5.1 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;

See alsoSection 5.6.1, CREATE VIEW, Section 5.6.4, DROP VIEW, Section 5.6.3, CREATE OR ALTER VIEW