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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 specified 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  trig-
       ger  can skip the operation for the current row, or change the row being inserted (for INSERT and UPDATE opera-
       tions only). If the trigger fires after the event, all changes, including the last insertion, update, or  dele-
       tion, 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.  Multi-
              ple 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 exe-
              cuted. 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. The user must also have
       EXECUTE privilege on the trigger function.

       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 currently missing:

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

       ? 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 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 user-defined function for the triggered action. The standard allows
         the execution of a number of other SQL commands, such as CREATE TABLE as the triggered action.  This  limita-
         tion 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 consistent. There is also unpredictable behavior when BEFORE  triggers  modify  rows  that  are
       later  to  be  modified by referential 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         2014-02-17                 CREATE TRIGGER(7)