Before Insert Trigger to check a value exists in database

Question

I would like to creating a trigger that updates the database when a new message is sent. If the SenderId and RecipentId already have a conversationId, then update the database so that the New.conversationId = existing conversationId.

Table: mailbox_message

Create a trigger that updates the conversationId to the same value whenever users 11 & 5 send messages to each other.

delimiter $$
create trigger before_insert_conversationId
before insert on data25k29_mailbox_message
for each row
begin
    IF ( EXISTS (
            SELECT data25k29_mailbox_message.conversationId FROM data25k29_mailbox_message
            WHERE  (data25k29_mailbox_message.senderId='5'
            and data25k29_mailbox_message.recipientId = '11')
            and (data25k29_mailbox_message.senderId= '11'
            and data25k29_mailbox_message.recipientId= '5')
                )
        )    
    THEN
        SET NEW.conversationId = data25k29_mailbox_message.conversationId;
    ELSE
        SET NEW.conversationId = NEW.conversationId;
    END IF;
end 
$$

The value doesn't change after inserting values. Any help in this regard is greatly appreciated. Thanks

Answer 1

This is my working snippet of this question.

Drop trigger if exists after_insert_conversationId;
delimiter |
CREATE TRIGGER after_insert_conversationId
before insert ON data25k29_mailbox_message
FOR EACH ROW

BEGIN
 Declare v1 int;
 select conversationId into v1 from data25k29_mailbox_message
 where senderId = new.senderId and recipientId = new.recipientId  limit 1;

 if v1 is NULL
then
set new.conversationId = new.conversationId;

else
set new.conversationId = v1;

End if;
END|