How to Make A Trigger Before Insert In Postgresql?

To create a trigger before insert in PostgreSQL, you need to define a trigger function that will be executed before any insert operation on a specific table. This trigger function can be written in PL/pgSQL language and using the CREATE FUNCTION statement. Inside the trigger function, you can implement the desired logic or validation that should be performed before the insert operation is executed. Once the trigger function is defined, you can create a trigger using the CREATE TRIGGER statement, specifying the event (BEFORE INSERT), the trigger function to be executed, and the table on which the trigger should be applied. This way, whenever an insert operation is attempted on the specified table, the trigger function will be executed beforehand, allowing you to control the data being inserted or perform any necessary pre-processing.

What is the order of execution of triggers in PostgreSQL?

In PostgreSQL, triggers are processed in the following order:

1. BEFORE triggers on the triggering event are fired.
2. The triggering event is executed.
3. AFTER triggers on the triggering event are fired.

Within each of these categories, triggers are executed in the order in which they were defined on the table.

How to set a specific order for trigger execution in PostgreSQL?

To set a specific order for trigger execution in PostgreSQL, you can use the CREATE TRIGGER statement with the BEFORE or AFTER keyword to specify when the trigger should be executed. You can also use the FOR EACH ROW or FOR EACH STATEMENT clause to define whether the trigger should be fired for each row affected by the triggering event.

For example, to create a trigger that fires before an update operation and another trigger that fires after the update operation, you can use the following syntax:

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CREATE TRIGGER trigger1
BEFORE UPDATE ON table_name
FOR EACH ROW
EXECUTE PROCEDURE trigger_function();

CREATE TRIGGER trigger2
AFTER UPDATE ON table_name
FOR EACH ROW
EXECUTE PROCEDURE trigger_function();

In this example, trigger1 will be executed before the update operation, while trigger2 will be executed after the update operation. By specifying the order and timing of the triggers, you can control the sequence in which they are executed.

What is the syntax for creating a trigger in PostgreSQL?

The syntax for creating a trigger in PostgreSQL is as follows:

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CREATE [ CONSTRAINT ] TRIGGER trigger_name
{ BEFORE | AFTER | INSTEAD OF } { event1 [ OR ... ] }
ON table_name
[ FROM referenced_table_name ]
[ NOT DEFERRABLE | [ DEFERRABLE ] [ INITIALLY IMMEDIATE | INITIALLY DEFERRED ] ]
[ REFERENCING { { NEW | OLD } TABLE [ AS ] transition_relation_name } [ ... ] ]
[ FOR [ EACH ] { ROW | STATEMENT } ]
[ WHEN ( condition ) ]
EXECUTE FUNCTION function_name ( arguments )

Here's an example illustrating the syntax:

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CREATE TRIGGER update_timestamp
BEFORE UPDATE ON employees
FOR EACH ROW
EXECUTE FUNCTION update_modified_timestamp();

In this example, a trigger named update_timestamp is defined to fire before an UPDATE operation on the employees table. The trigger calls the update_modified_timestamp function for each row that is being updated.