CREATE TRIGGER(7)                                         SQL Commands                                         CREATE TRIGGER(7)



NAME
       CREATE TRIGGER - define a new trigger


SYNOPSIS
       CREATE TRIGGER name { BEFORE | AFTER } { event [ OR ... ] }
           ON table [ FOR [ EACH ] { ROW | STATEMENT } ]
           EXECUTE PROCEDURE funcname ( arguments )


DESCRIPTION
       CREATE TRIGGER creates a new trigger. The trigger will be associated with the specified table and will execute the speci-
       fied function funcname when certain events occur.

       The trigger can be specified to fire either before the operation is attempted on a row (before  constraints  are  checked
       and  the  INSERT, UPDATE, or DELETE is attempted) or after the operation has completed (after constraints are checked and
       the INSERT, UPDATE, or DELETE has completed). If the trigger fires before the event, the trigger can skip  the  operation
       for the current row, or change the row 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 is marked FOR EACH ROW 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 row. In contrast, a trigger that is marked FOR EACH STATEMENT 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).

       In addition, triggers may be defined to fire for a TRUNCATE, though only FOR EACH STATEMENT.

       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 cannot create SELECT triggers. Rules and views are more appropriate in such cases.

       Refer to in the documentation for more information about triggers.

PARAMETERS
       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, UPDATE, DELETE, or TRUNCATE; 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.

       funcname
              A user-supplied function that is declared as taking no arguments and returning type  trigger,  which  is  executed
              when the trigger fires.

       arguments
              An  optional  comma-separated  list  of arguments to be provided to the function when the trigger is executed. The
              arguments are literal string constants. Simple names and numeric constants can be written here, too, but they will
              all  be  converted to strings. Please check the description of the implementation language of the trigger function
              about how the trigger arguments are accessible within the function; it might be  different  from  normal  function
              arguments.

NOTES
       To create a trigger on a table, the user must have the TRIGGER privilege on the table.

       Use DROP TRIGGER [drop_trigger(7)] to remove a trigger.

       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.

EXAMPLES
       in the documentation contains a complete example.

COMPATIBILITY
       The  CREATE  TRIGGER statement in PostgreSQL implements a subset of the SQL standard. The following functionality is cur-
       rently missing:

       o SQL allows triggers to fire on updates to specific columns (e.g., AFTER UPDATE OF col1, col2).

       o SQL 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 Post-
         greSQL allows trigger procedures to be written in any number of user-defined languages, access to the data  is  handled
         in a language-specific way.

       o PostgreSQL  only allows the execution of a user-defined function for the triggered action. The standard allows the exe-
         cution of a number of other SQL commands, such as CREATE TABLE as the triggered action. This limitation is not hard  to
         work around by creating a user-defined function that executes the desired commands.

       SQL  specifies  that  multiple  triggers should be fired in time-of-creation order. PostgreSQL uses name order, which was
       judged to be more convenient.

       SQL specifies that BEFORE DELETE triggers on cascaded deletes fire after the cascaded DELETE completes.   The  PostgreSQL
       behavior is for BEFORE DELETE to always fire before the delete action, even a cascading one. This is considered more con-
       sistent. There is also unpredictable behavior when BEFORE triggers modify rows that are later to be modified by  referen-
       tial actions. This can lead to constraint violations or stored data that does not honor the referential constraint.

       The ability to specify multiple actions for a single trigger using OR is a PostgreSQL extension of the SQL standard.

       The ability to fire triggers for TRUNCATE is a PostgreSQL extension of the SQL standard.

SEE ALSO
       CREATE FUNCTION [create_function(7)], ALTER TRIGGER [alter_trigger(7)], DROP TRIGGER [drop_trigger(7)]



SQL - Language Statements                                  2011-09-22                                          CREATE TRIGGER(7)