Summary
A trigger is a binding of a set of actions to certain operations performed on a table or view. This set of actions is defined in a special trigger function which is distinguished by specifying the type of the returned value to be of a special pseudotype trigger. So, each time an operation (INSERT
, UPDATE
, DELETE
, or TRUNCATE
) is performed on the table, this trigger function is called by the system.
It can be executed either for each row or for each statement. If executed for each row (row-level trigger), the function is passed special variables such as OLD
and NEW
.
This will contain the row's contents, as it is currently in the database (OLD
) and as it is the moment the trigger function is called (NEW
). Where the OLD
or NEW
value is missing, ...
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