CURSOR in a PROCEDURE that have a sub query

Question

I have this procedure in MySQL (a part of it)

DECLARE num_rows INT(11) DEFAULT NULL;
DECLARE insert_result INT(11) DEFAULT NULL;

DECLARE var_user_id INT DEFAULT NULL;
DECLARE user_that_posted_this_post INT DEFAULT NULL;
DECLARE arg_in_group_id INT DEFAULT NULL;
DECLARE how_many_comments INT DEFAULT 0;

/* Cursor 1 (posts) */
DECLARE done INT DEFAULT 0;
DECLARE c1 CURSOR FOR 
    SELECT user_id 
    FROM user_rights 
    WHERE user_rights.right = 101 AND 
    user_rights.group_id  = 
    (
        SELECT 
        posted_in
        FROM posts
        WHERE
        id = arg_post_id
    )
    ORDER BY user_id DESC;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;

/* See how many comments has this post */
SELECT count(id) FROM  comments_posts WHERE posted_in = arg_post_id INTO how_many_comments;

/* Fetch the autor of this post and the group_id */
SELECT posted_by,posted_in FROM posts WHERE id = arg_post_id INTO user_that_posted_this_post,arg_in_group_id;

IF arg_post_id IS NULL OR arg_post_id = ''
THEN
    SELECT '0' AS response;
ELSE
    DELETE FROM posts WHERE id = arg_post_id;
END IF;

/* Increment the notifications for every afected user */
INSERT INTO t VALUES(0);
OPEN c1;
read_loop: LOOP
    FETCH c1 INTO var_user_id;
        IF done THEN
            LEAVE read_loop;
        ELSE
            INSERT INTO t VALUES(var_user_id);
        END IF;
END LOOP;
CLOSE c1;

and i have some wired things happening (cursor is not taking any values), for example this query

SELECT user_id 
FROM user_rights 
WHERE user_rights.right = 101 AND 
user_rights.group_id  = 
(
    SELECT 
    posted_in
    FROM posts
    WHERE
    id = arg_post_id
)

returns all users that have right 101 (read) in the group that a post has been posted. The result works perfectly in sql, for example

SELECT user_id 
FROM user_rights 
WHERE user_rights.right = 101 AND 
user_rights.group_id  = 
(
    SELECT 
    posted_in
    FROM posts
    WHERE
    id = 247
)

RESULT
user_id
1
3
2
16
17
20
19  

but when i try to output from the CURSOR

/* Increment the notifications for every afected user */
INSERT INTO t VALUES(0);
OPEN c1;
read_loop: LOOP
    FETCH c1 INTO var_user_id;
        IF done THEN
            LEAVE read_loop;
        ELSE
            INSERT INTO t VALUES(var_user_id);
        END IF;
END LOOP;
CLOSE c1;

it does not work ... it only writes "0" in t table why is that? I can't use sub query in a cursor?

Answer 1

It dont work because the cursor is asensitive, meaning that it points at the real data. it dont work with subqueries like this:

DECLARE c1 CURSOR FOR 
    SELECT user_id 
    FROM user_rights 
    WHERE user_rights.right = 101 AND 
    user_rights.group_id  = 
    (
        SELECT 
        posted_in
        FROM posts
        WHERE
        id = arg_post_id
    )
    ORDER BY user_id DESC;

Answer 2

As far as I know, a cursor cant be declarated with subquery.