CREATE TRIGGER name { BEFORE | AFTER } { event [OR ...] }
ON table FOR EACH { ROW | STATEMENT }
EXECUTE PROCEDURE func ( arguments )
|
CREATE TRIGGER will enter a new trigger into the current database. The trigger will be associated with the relation table and will execute the specified function func. Only the relation owner may create a trigger on this relation.
You can specify the trigger to fire either before BEFORE the operation is attempted on a tuple (before constraints are checked and the INSERT, UPDATE or DELETE is attempted) or AFTER the operation has been attempted (for example, after constraints are checked and the INSERT, UPDATE or DELETE has completed). If the trigger fires before the event, the trigger may skip the operation for the current tuple, or change the tuple being inserted (for INSERT and UPDATE operations only). If the trigger fires after the event, all changes, including the last insertion, update, or deletion, are "visible" to the trigger.
SELECT does not modify any rows, so you cannot create SELECT triggers. Rules and views are more appropriate in such cases.
See DROP TRIGGER for information on how to remove triggers.
See CREATE FUNCTION for information on how to register a user-supplied function.
To create a trigger on a table, the user must have the TRIGGER privilege on the table.
As of the current release, STATEMENT triggers are not implemented.
Refer to the Red Hat Database Programmer's Guide for more information on trigger procedures.
Check if the specified distributor code exists in the distributors table before appending or updating a row in the table films:
CREATE TRIGGER if_dist_exists
BEFORE INSERT OR UPDATE ON films FOR EACH ROW
EXECUTE PROCEDURE check_primary_key ('did', 'distributors', 'did'); |
Before canceling a distributor or updating its code, remove every reference to the table films:
CREATE TRIGGER if_film_exists
BEFORE DELETE OR UPDATE ON distributors FOR EACH ROW
EXECUTE PROCEDURE check_foreign_key
(1, 'CASCADE', 'did', 'films', 'did'); |
The second example can also be done by using a foreign key, constraint as in:
CREATE TABLE distributors (
did DECIMAL(3),
name VARCHAR(40),
CONSTRAINT if_film_exists
FOREIGN KEY(did) REFERENCES films
ON UPDATE CASCADE ON DELETE CASCADE
); |
The CREATE TRIGGER statement in PostgreSQL implements a subset of the SQL99 standard. The following functionality is missing:
SQL99 allows triggers to fire on updates to specific columns (for example, AFTER UPDATE OF col1, col2).
SQL99 allows you to define aliases for the "old" and "new" rows or tables for use in the definition of the triggered action (for example, CREATE TRIGGER ... ON tablename REFERENCING OLD ROW AS somename NEW ROW AS othername ...). As PostgreSQL allows trigger procedures to be written in any number of user-defined languages, access to the data is handled in a language-specific way.
PostgreSQL only has row-level triggers, no statement-level triggers.
PostgreSQL only allows the execution of a stored procedure for the triggered action. SQL99 allows the execution of a number of other SQL commands, such as CREATE TABLE as triggered action. This limitation is not hard to work around by creating a stored procedure that executes these commands.