CREATE TRIGGER SQL - Language Statements CREATE TRIGGER define a new trigger 2000-03-25 CREATE TRIGGER name { BEFORE | AFTER } { event [ OR ... ] } ON table [ FOR [ EACH ] { ROW | STATEMENT } ] EXECUTE PROCEDURE func ( arguments ) 1998-09-21 name The name to give the new trigger. This must be distinct from the name of any other trigger for the same table. BEFORE AFTER Determines whether the function is called before or after the event. event One of INSERT, DELETE or UPDATE; this specifies the event that will fire the trigger. Multiple events can be specified using OR. table The name (optionally schema-qualified) of the table the trigger is for. FOR EACH ROW FOR EACH STATEMENT This specifies whether the trigger procedure should be fired once for every row affected by the trigger event, or just once per SQL statement. If neither is specified, FOR EACH STATEMENT is the default. func A user-supplied function that is declared as taking no arguments and returning type trigger. arguments An optional comma-separated list of arguments to be provided to the function when the trigger is executed, along with the standard trigger data such as old and new tuple contents. The arguments are literal string constants. Simple names and numeric constants may be written here too, but they will all be converted to strings. Note that these arguments are not provided as normal function parameters (since a trigger procedure must be declared to take zero parameters), but are instead accessed through the TG_ARGV array. 1998-09-21 CREATE TRIGGER This message is returned if the trigger is successfully created. 1998-09-21 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. The trigger can be specified 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 (e.g., 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. A trigger that executes FOR EACH ROW of the specified operation is called once for every row that the operation modifies. For example, a DELETE that affects 10 rows will cause any ON DELETE triggers on the target relation to be called 10 separate times, once for each deleted tuple. In contrast, a trigger that executes FOR EACH STATEMENT of the specified operation only executes once for any given operation, regardless of how many rows it modifies (in particular, an operation that modifies zero rows will still result in the execution of any applicable FOR EACH STATEMENT triggers). If multiple triggers of the same kind are defined for the same event, they will be fired in alphabetical order by name. SELECT does not modify any rows so you can not create SELECT triggers. Rules and views are more appropriate in such cases. Refer to for more information. To create a trigger on a table, the user must have the TRIGGER privilege on the table. In PostgreSQL versions before 7.3, it was necessary to declare trigger functions as returning the placeholder type opaque, rather than trigger. To support loading of old dump files, CREATE TRIGGER will accept a function declared as returning opaque, but it will issue a NOTICE and change the function's declared return type to trigger. Refer to the command for information on how to remove triggers. 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 cancelling 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 ); SQL92 There is no CREATE TRIGGER statement in SQL92. SQL99 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 (e.g., 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 (e.g., CREATE TRIGGER ... ON tablename REFERENCING OLD ROW AS somename NEW ROW AS othername ...). Since 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 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. SQL99 specifies that multiple triggers should be fired in time-of-creation order. PostgreSQL uses name order, which was judged more convenient to work with. The ability to specify multiple actions for a single trigger using OR is a PostgreSQL extension of the SQL standard.