CREATE VIEW

<< VIEW | FB 2.5 Language Reference | ALTER VIEW >>

CREATE VIEW

Available in: DSQL, ESQL

Syntax

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

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


Views can select from stored procedures

Changed in: 2.5

Description

In Firebird 2.5 and up, views can select from selectable stored procedures.

Example

 create view low_bones as
   select id, name, description from them_bones('human') 
   where name in ('leg_bone', 'foot_bone', 'toe_bone')


Views can infer column names from derived tables or GROUP BY

Changed in: 2.5

Description

In Firebird 2.5 and up, views can infer the names of columns from a derived table or involved in a GROUP BY clause. Previously it was necessary to specify explicit aliases for these columns (either per column or in a full list).

Examples

 create view tickle as
   select t from (select t from tackle)

 create view vstocks as
   select kind, sum(stock) s from stocks
   group by kind

In the second example, notice that it is still necessary to alias the SUM column. Previous Firebird versions also required an explicit alias for the KIND column.

Per-column aliases supported in view definition

Changed in: 2.1

Description

Firebird 2.1 and up allow the use of column aliases in the SELECT statement. You can alias none, some or all of the columns; each alias used becomes the name of the corresponding view column.

Syntax (partial)

 CREATE VIEW viewname [<full_column_list>]
   AS
   SELECT <column_def> [, <column_def> ...]
   FROM ...
   [WITH CHECK OPTION]

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

 <column_def> ::= {source_col | expr} [[AS] colalias]

Notes:

  • If the full column list is also present, specifying column aliases is futile as they will be overridden by the names in the column list.
  • The full column list used to be mandatory for views whose SELECT statement contains expression-based columns or identical column names. Now you can omit the full column list, provided that you alias the offending columns in the SELECT clause.

back to top of page

Full SELECT syntax supported

Changed in: 2.0

Description

From Firebird 2.0 onward view definitions are considered full-fledged SELECT statements. Consequently, the following elements are (re)allowed in view definitions: FIRST, SKIP, ROWS, ORDER BY, PLAN and UNION.

Note: In Firebird 2.5 and up, it is no longer necessary to supply a view column list if the view is based on a UNION:

 create view vplanes (make, model) as
  select make, model from jets
    union
  select make, model from props
    union
  select make, model from gliders

The column names will be taken from the union. Of course you can still override them with a view column list.

PLAN subclause disallowed in 1.5, reallowed in 2.0

Changed in: 1.5, 2.0

Description

Firebird versions 1.5.x forbid the use of a PLAN subclause in a view definition. From 2.0 onward a PLAN is allowed again.

back to top of page

Triggers on updatable views block auto-writethrough

Changed in: 2.0

Description

In versions prior to 2.0, Firebird often did not block the automatic writethrough to the underlying table if one or more triggers were defined on a naturally updatable view. This could cause mutations to be performed twice unintentionally, sometimes leading to data corruption and other mishaps. Starting at Firebird 2.0, this misbehaviour has been corrected: now if you define a trigger on a naturally updatable view, no mutations to the view will be automatically passed on to the table; either the trigger takes care of that, or nothing will. This is in accordance with the description in the InterBase 6 Data Definition Guide under Updating views with triggers.

Warning: Some people have developed code that takes advantage of the previous behaviour. Such code should be corrected for Firebird 2.0 and higher, or mutations may not reach the table at all.

View with non-participating NOT NULL columns in base table can be made insertable

Changed in: 2.0

Description

Any view whose base table contains one or more non-participating NOT NULL columns is read-only by nature. It can be made updatable by the use of triggers, but even with those, all INSERT attempts into such views used to fail because the NOT NULL constraint on the base table was checked before the view trigger got a chance to put things right. In Firebird 2.0 and up this is no longer the case: provided the right trigger is in place, such views are now insertable.

Example

The view below would give validation errors for any insert attempts in Firebird 1.5 and earlier. In Firebird 2.0 and up it is insertable:

 create table base (x int not null, y int not null);

 create view vbase as select x from base;

 set term #;
 create trigger bi_base for vbase before insert
 as
 begin
   if (new.x is null) then new.x = 33;
   insert into base values (new.x, 0);
 end#
 set term ;#

Notes:

  • Please notice that the problem described above only occured for NOT NULL columns that were left outside the view.
  • Oddly enough, the problem would be gone if the base table itself had a trigger converting NULL input to something valid. But then there was a risk that the insert would take place twice, due to the auto-writethrough bug that has also been fixed in Firebird 2.

See also:
View
CREATE TABLE
DROP VIEW
GRANT
INSERT
REVOKE
SELECT
UPDATE
UNION
DDL - Data Definition Language

back to top of page
<< VIEW | FB 2.5 Language Reference | ALTER VIEW >>