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ALTER TABLE
ADD column: Context variables as defaults
Changed in: IB
Description:
Any context variable that is assignment-compatible to the new column's data type can be used as a default. This was already the case in InterBase 6, but the Language Reference only mentioned USER.
Example
alter table MyData add MyDay date default current_date
ALTER COLUMN also for generated (computed) columns
Available in: DSQL
Added in: 2.5
Description
Firebird 2.5 supports the altering of generated (computed) columns, something that was previously impossible. Only the data type and the generation expression can be changed; you cannot change a base column into a generated column or vice versa.
Syntax
ALTER TABLE tablename ALTER [COLUMN] gencolname [TYPE datatype] {GENERATED ALWAYS AS | COMPUTED BY} (expression)
Example
create table nums (a int, b generated always as (3*a)); commit; alter table nums alter b generated always as (4*a + 7); commit;
Notice that you can use GENERATED ALWAYS AS when altering columns defined with COMPUTED BY and vice versa.
ALTER COLUMN ... TYPE no longer fails if column is used in trigger or SP
Changed in: 2.5
Description
Previously, if a table column was referenced in a stored procedure or trigger, the column's type could not be changed, even if the change would not break the PSQL code. Now such changes are permitted – even if they do break the code.
Warning: This means that, in the current situation, you can commit changes that break SP's or triggers without getting as much as a warning! For information on how to track down invalidated PSQL modules after a column type change, please read the note The RDB$VALID_BLR field, near the end of this document.
ALTER COLUMN: DROP DEFAULT
Available in: DSQL
Added in: 2.0
Description
Firebird 2 adds the possibility to drop a column-level default. Once the default is dropped, there will either be no default in place or – if the column's type is a DOMAIN with a default – the domain default will resurface.
Syntax
ALTER TABLE tablename ALTER [COLUMN] colname DROP DEFAULT
Example
alter table Trees alter Girth drop default
An error is raised if you use DROP DEFAULT on a column that doesn't have a default or whose effective default is domain-based.
ALTER COLUMN: SET DEFAULT
Available in: DSQL
Added in: 2.0
Description
Firebird 2 adds the possibility to set/alter defaults on existing columns. If the column already had a default, the new default will replace it. Column-level defaults always override domain-level defaults.
Syntax
ALTER TABLE tablename ALTER [COLUMN] colname SET DEFAULT <default> <default> ::= literal-value | context-variable | NULL
Example
alter table Customers alter EnteredBy set default current_user
Tip: If you want to switch off a domain-based default on a column, set the column default to NULL.
back to top of page ALTER COLUMN: POSITION now 1-based Changed in: 1.0
Description
When changing a column's position, the engine now interprets the new position as 1-based. This is in accordance with the SQL standard and the InterBase documentation, but in practice InterBase interpreted the position as 0-based.
Syntax
ALTER TABLE tablename ALTER [COLUMN] colname POSITION <newpos> <newpos> ::= an integer between 1 and the number of columns
Example
alter table Stock alter Quantity position 3
Note: Don't confuse this with the POSITION in CREATE/ALTER TRIGGER. Trigger positions are and will remain 0-based.
CHECK accepts NULL outcome
Changed in: 2.0
Description
If a CHECK constraint resolves to NULL, Firebird versions before 2.0 reject the input. Following the SQL standard to the letter, Firebird 2.0 and above let NULLs pass and only consider the check failed if the outcome is false. For more information see under CREATE TABLE.
FOREIGN KEY without target column references PK
Changed in: IB
Description
If you create a foreign key without specifying a target column, it will reference the primary key of the target table. This was already the case in InterBase 6, but the IB Language Reference wrongly states that in such cases, the engine scans the target table for a column with the same name as the referencing column.
Example
create table eik ( a int not null primary key, b int not null unique ); create table beuk ( b int ); alter table beuk add constraint fk_beuk foreign key (b) references eik; -- beuk.b now references eik.a, not eik.b !
FOREIGN KEY creation no longer requires exclusive access
Changed in: 2.0
Description
In Firebird 2.0 and above, adding a foreign key constraint no longer requires exclusive access to the database.
GENERATED ALWAYS AS
Added in: 2.1
Description
Instead of COMPUTED [BY], you may also use the SQL-2003-compliant equivalent GENERATED ALWAYS AS for computed fields.
Syntax
colname [coltype] GENERATED ALWAYS AS (expression)
Example
alter table Friends add fullname varchar(74) generated always as (firstname || coalesce(' ' || middlename, '') || ' ' || lastname)
UNIQUE constraints now allow NULLs
Changed in: 1.5
Description
In compliance with the SQL-99 standard, NULLs – even multiple – are now allowed in columns with a UNIQUE constraint. For a full discussion, see CREATE TABLE :: UNIQUE constraints now allow NULLs.
USING INDEX subclause
Available in: DSQL
Added in: 1.5
Description
A USING INDEX subclause can be placed at the end of a primary, unique or foreign key definition. Its purpose is to:
- provide a user-defined name for the automatically created index that enforces the constraint, and
- optionally define the index to be ascending or descending (the default being ascending).
Syntax
[ADD] [CONSTRAINT constraint-name] <constraint-type> <constraint-definition> [USING [ASC[ENDING] | DESC[ENDING]] INDEX index_name]
For a full discussion and examples, see CREATE TABLE :: USING INDEX subclause.