How to insert trigger in PostgreSQL?

Question

I have two tables:

1. users
2. trainer

In the users table, I have a column is_trainer=false by default`.

When a user inputs the information in the table users and chooses the column is_trainer=true I want to insert the user information in the trainer table.

When I omit the if condition, after I insert a user in the users table it puts the information in the trainer table, but when I put if condition, it cannot work! I think it thinks that is_trainer is for the trainer table not for the users table

My code:

BEGIN
    IF NEW.is_trainer <> OLD.is_trainer THEN
        INSERT INTO trainer(trainer_id, sport_id, fee_per_hour, experience, created_by)
        VALUES(users.user_id, 3, '200$', 3, 6);
    END IF;
    RETURN NEW;
END;

Answer 1

This will not work as you expect:

IF NEW.is_trainer <> OLD.is_trainer THEN

In an INSERT trigger, there is not OLD value, ie all columns in pseudo table OLD are NULL . So your expression boils down to:

IF NEW.is_trainter <> NULL THEN

This condition can never be true, since nothing is different that NULL , nor equal to NULL (to check for nullness, you need IS NULL ).

Based on your description of your requirement, I think that you want:

IF NEW.is_traiter = true THEN

Also, your INSERT command will not work, since it is referencing unknown relation users . You probably want:

INSERT INTO trainer(trainer_id,sport_id,fee_per_hour,experience,created_by)
VALUES(NEW.user_id, 3, '200$', 3, 6);

Likely, other values that you are giving for insert into trainer should also come from pseudo-table NEW (such as created_by for example).