Forcing to first update then select in a procedure

Question

I have this procedure in MySQL (don't bother to figure it out what it does, just watch the part where the cursor is opened)

/* PROCEDURE 1 : Post notification */
DROP PROCEDURE IF EXISTS AddNotificationOnPosts;

DELIMITER $$
CREATE DEFINER=`root`@`localhost` PROCEDURE `AddNotificationOnPosts`(arg_from_user INT(11),arg_on_post_id INT(11),arg_in_group_id INT(11))
BEGIN
    DECLARE num_rows INT DEFAULT NULL;
    DECLARE insert_result INT DEFAULT NULL;

    DECLARE done INT DEFAULT 0;
    DECLARE var_user_id INT DEFAULT NULL;
        DECLARE c1 CURSOR FOR 
        SELECT user_id 
        FROM user_rights 
        WHERE user_rights.right = 101 AND user_rights.group_id  = arg_in_group_id 
        ORDER BY user_id DESC;
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;

    IF(arg_from_user IS NULL OR arg_from_user = '')
    THEN
        SELECT "0" AS response;
    ELSEIF(arg_on_post_id IS NULL OR arg_on_post_id = '')
    THEN
        SELECT "0" AS response;
    ELSEIF(arg_in_group_id IS NULL OR arg_in_group_id = '')
    THEN
        SELECT "0" AS response;
    ELSE
        SELECT count(notification_id) FROM notifications_posts 
        WHERE 
        from_user = arg_from_user AND
        on_post_id = arg_on_post_id AND
        in_group_id = arg_in_group_id
        INTO num_rows;
    END IF;

    IF num_rows = 0
    THEN
        INSERT INTO notifications_posts(from_user,on_post_id,in_group_id) VALUES(arg_from_user,arg_on_post_id,arg_in_group_id);
        SELECT ROW_COUNT() INTO insert_result;

        IF insert_result > 0 
        THEN        
            /* Increment the notifications for every user*/
            OPEN c1;
            read_loop: LOOP
                FETCH c1 INTO var_user_id;
                    IF done THEN
                        LEAVE read_loop;
                    ELSE
                        IF NOT(var_user_id = arg_from_user)
                        THEN

                            /* UPDATING */
                            UPDATE user_info SET notifications = notifications + 1 WHERE user_info.user_id = var_user_id;

                            /* SELECTING RESULT */
                            SELECT
                            user_info.user_id,
                            messages,
                            tasks,
                            notifications,
                            messages+tasks+notifications AS total
                            FROM user_rights
                            INNER
                                JOIN user_info
                                ON user_info.user_id = user_rights.user_id
                            WHERE user_rights.right = 101 AND user_rights.group_id  = arg_in_group_id ;

                        END IF;
                    END IF;
            END LOOP;
            CLOSE c1;


        ELSE
            SELECT "0" AS response;
        END IF;

    ELSE
        SELECT "0" AS response;
    END IF;
END $$
DELIMITER ;

and in this part

            /* Increment the notifications for every user*/
            OPEN c1;
            read_loop: LOOP
                FETCH c1 INTO var_user_id;
                    IF done THEN
                        LEAVE read_loop;
                    ELSE
                        IF NOT(var_user_id = arg_from_user)
                        THEN

                            /* UPDATING */
                            UPDATE user_info SET notifications = notifications + 1 WHERE user_info.user_id = var_user_id;

                            /* SELECTING RESULT */
                            SELECT
                            user_info.user_id,
                            messages,
                            tasks,
                            notifications,
                            messages+tasks+notifications AS total
                            FROM user_rights
                            INNER
                                JOIN user_info
                                ON user_info.user_id = user_rights.user_id
                            WHERE user_rights.right = 101 AND user_rights.group_id  = arg_in_group_id ;

                        END IF;
                    END IF;
            END LOOP;
            CLOSE c1;

the procedure selects data, then update it (believe me, i'm dead serious), and i really need to select after, because i'm generating an XML with the altered data, so:

1 Why is selecting then updating, because i see that select is underneath the update

2 Who had this "brilliant idea" to make it this way?

Thank you.

Answer 1

When you've got in one batch update/delete and something else in sql-server, it is completely indeterministic in which order "commands" will be execute. The solution to this problem is to put "update" in different batch, by using "exec" statement. So try use:

exec('UPDATE user_info SET notifications = notifications + 1 WHERE user_info.user_id =' + var_user_id); 

It should work now.

Answer 2

I solved the problem: the SELECT statement must be outside of the CURSOR LOOP.