Question

I am requiring that the user’s current password cannot match the last five used passwords. The expired passwords will be stored in a table similar to this.

################################################################
#                             table_A                          #
################################################################
# id #     label    # attr_string #    attr_datetime    # u_id #
################################################################
#  1 # expired_pass #  some_hash  # YYYY-MM-DD HH:MM:SS #  1   #
#  2 # expired_pass #  some_hash  # YYYY-MM-DD HH:MM:SS #  1   #
#  3 # expired_pass #  some_hash  # YYYY-MM-DD HH:MM:SS #  1   #
#  4 # expired_pass #  some_hash  # YYYY-MM-DD HH:MM:SS #  1   #
#  5 # expired_pass #  some_hash  # YYYY-MM-DD HH:MM:SS #  1   #
################################################################

Now I am wanting a trigger that upon input checks if there are >= 5 expired passwords already stored if so then remove the oldest password. This is what I have come up with so far.

DELIMITER //
    CREATE TRIGGER removePass AFTER INSERT ON `user_attributes`(
        SET @user_id := SELECT u_id FROM `user_attributes` WHERE id = LAST_INSERT_ID()//
        SET @num := SELECT COUNT(id) FROM `user_attributes` WHERE u_id = @user_id AND label = 'expired_password'//
        IF @num >= 5 THEN
            DELETE FROM `user_attributes`
            WHERE id IN(
                SELECT id
                FROM `user_attributes`
                WHERE u_id = @user_id
                AND label = 'expired_pass'
                ORDER BY attr_datetime DESC
                LIMIT 1

            )
        END IF
    )//
DELIMITER ;

Answer 1

You can avoid using attr_datetime and rely on ids:

DELETE FROM `user_attributes`
            WHERE id = (
                SELECT MIN(id)
                FROM `user_attributes`
                WHERE u_id = @user_id
                AND label = 'expired_pass'
            )

P.S. Thank to @sixlettervariables for the comment and Brock for advice.