MySQL Stored Procedure Nested Cursors Error - mysql

Question

i am copying data for a Many to Many Relationship table making a Third Table.
The stored procedure currently looks like this but it has some error
DELIMITER $$
CREATE PROCEDURE `test`.UpdateRelatedAccounts()
BEGIN
DECLARE ssn_sel_id VARCHAR(255) DEFAULT 0;
DECLARE id_sel_id CHAR(36) DEFAULT 0;
DECLARE id_sel_rel CHAR(36) DEFAULT 0;
DECLARE no_more_rows BOOLEAN;
DECLARE num_rows INT DEFAULT 0;
DECLARE no_more_rel_rows BOOLEAN;
DECLARE rel_num_rows INT DEFAULT 0;
DECLARE ssn_all_cur CURSOR FOR
SELECT ssn, id FROM ssn WHERE ssn NOT IN ('','000-00-0000');
DECLARE ssn_cur CURSOR FOR
SELECT id FROM ssn WHERE id != id_sel_id AND ssn = ssn_sel_id;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET no_more_rows = TRUE;
-- DECLARE CONTINUE HANDLER FOR NOT FOUND SET no_more_rel_rows = TRUE;
OPEN ssn_all_cur;
SELECT FOUND_ROWS() INTO num_rows;
the_loop: LOOP
FETCH ssn_all_cur
INTO ssn_sel_id, id_sel_id;
IF no_more_rows THEN
CLOSE ssn_all_cur;
LEAVE the_loop;
END IF;
OPEN ssn_cur;
SELECT FOUND_ROWS() INTO rel_num_rows;
the_rel_loop: LOOP
FETCH ssn_cur
INTO id_sel_rel;
IF no_more_rel_rows THEN
CLOSE ssn_cur;
LEAVE the_rel_loop;
END IF;
INSERT INTO `ssn_related` ( `ssn_primary`, `ssn_related` ) VALUES ( id_sel_id, id_sel_rel ), ( id_sel_rel, id_sel_id );
END LOOP the_rel_loop;
END LOOP the_loop;
END$$
DELIMITER ;
How do i nest to use the values and insert into third table.

Answer 1

Cursors are slow and most of the time unnecessary. Nested cursors are slow² and unnecessary². Cursors shall only be used as the last resort, when there's really no other way.
What you want to do can be broken down to this:
INSERT INTO `ssn_related` ( `ssn_primary`, `ssn_related` )
SELECT
ssn_1.id,
ssn_2.id
FROM
ssn ssn_1
INNER JOIN ssn ssn_2 ON ssn_2.ssn = ssn_1.id
WHERE ssn_1.ssn NOT IN ('', '000-00-0000')
AND ssn_2.id != ssn_1.id;
Then you do the same again with swapped columns in the SELECT.
INSERT INTO `ssn_related` ( `ssn_primary`, `ssn_related` )
SELECT
ssn_2.id,
ssn_1.id
FROM
ssn ssn_1
INNER JOIN ssn ssn_2 ON ssn_2.ssn = ssn_1.id
WHERE ssn_1.ssn NOT IN ('', '000-00-0000')
AND ssn_2.id != ssn_1.id;