PostgreSQL BEFORE UPDATE Trigger

Summary: in this tutorial, you will learn how to define a PostgreSQL BEFORE UPDATE trigger that executes a function before an update event occurs.

Introduction to the PostgreSQL BEFORE UPDATE trigger

In PostgreSQL, a trigger is a database object that is automatically activated when an event such as INSERT, UPDATE, DELETE, or TRUNCATE occurs on the associated table.

A BEFORE UPDATE trigger is a type of trigger that activates before an UPDATE operation is applied to a table.

These BEFORE UPDATE triggers can be particularly useful when you want to modify data before an update occurs or enforce certain conditions.

In a BEFORE UPDATE trigger, you can access the following variables:

OLD: This record variable allows you to access the row before the update.
NEW: This record variable represents the row after the update.

Also, you can access the following variables:

TG_NAME: Represent the name of the trigger.
TG_OP: Represent the operation that activates the trigger, which is UPDATE for the BEFORE UPDATE triggers.
TG_WHEN: Represent the trigger timing, which is BEFORE for the BEFORE UPDATE triggers.

To create a BEFORE UPDATE trigger, you follow these steps:

First, define a trigger function that will execute when the BEFORE UPDATE trigger fires:

CREATE OR REPLACE FUNCTION trigger_function()
   RETURNS TRIGGER
   LANGUAGE PLPGSQL
AS
$$
BEGIN
   -- trigger logic
   -- ...
   RETURN NEW;
END;
$$

Second, create a BEFORE UPDATE trigger that executes the defined function:

CREATE TRIGGER trigger_name
BEFORE UPDATE
ON table_name
FOR EACH {ROW | STATEMENT}
EXECUTE FUNCTION trigger_function();

PostgreSQL BEFORE UPDATE trigger example

First, create a new table called employees to store the employee data:

CREATE TABLE employees (
    id SERIAL PRIMARY KEY,
    name VARCHAR(100) NOT NULL,
    salary NUMERIC NOT NULL
);

Next, define a trigger function that raises an exception if the new salary is lower than the current salary. The trigger will prevent the update when the exception occurs.

CREATE OR REPLACE FUNCTION fn_before_update_salary()
RETURNS TRIGGER AS $$
BEGIN
    IF NEW.salary < OLD.salary THEN
        RAISE EXCEPTION 'New salary cannot be less than current salary';
    END IF;
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

Then, create a BEFORE UPDATE trigger that executes the fn_before_update_salary() before the update:

CREATE TRIGGER before_update_salary_trigger
BEFORE UPDATE OF salary ON employees
FOR EACH ROW
EXECUTE FUNCTION fn_before_update_salary();

This BEFORE UPDATE trigger ensures that the salary of the employee cannot be decreased. If you attempt to reduce the salary, the trigger will raise an exception and abort the update.

After that, insert some rows into the employees table:

INSERT INTO employees(name, salary)
VALUES
   ('John Doe', 70000),
   ('Jane Doe', 80000)
RETURNING *;

Output:

id |   name   | salary
----+----------+--------
  1 | John Doe |  70000
  2 | Jane Doe |  80000
(2 rows)

Finally, attempt to decrease the salary of John Doe:

UPDATE employees
SET salary = salary * 0.9
WHERE id = 1;

The BEFORE UPDATE trigger raises the following exception:

ERROR:  New salary cannot be less than current salary
CONTEXT:  PL/pgSQL function fn_before_update_salary() line 4 at RAISE

Summary

Use a BEFORE UPDATE trigger to automatically execute a function before an update.