mysql - Using Cursor in a Loop of a stored procedure

Question

So as to use cursors dynamically using MySQL is it possible to declare a cursor in a loop of a stored procedure? I've tried and got an error:

increment: LOOP
DECLARE cur1 CURSOR FOR SELECT person_id, publication_id  FROM p_publication WHERE person_id = new_count;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;
OPEN cur1;
REPEAT
    FETCH cur1 INTO pub_id, per_id;
IF NOT done THEN
          INSERT INTO test.t2 VALUES (pub_id, per_id);
    END IF;
SET new_count = new_count + 1;
  UNTIL done END REPEAT;

     CLOSE cur1;
IF !(new_count < old_count ) THEN LEAVE increment;
END IF;
END LOOP increment;

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'DECLARE cur1 CURSOR FOR SELECT person_id, publication_id FROM person_has_public' at line 12

Thanks In advance

Answer 1

You can do what you want by nesting the cursor in a BEGIN ... END block. See Roland Bouman's "Nesting MySQL Cursor Loops" article for more details. It may be worth noting his comments about this technique often being unnecessary, as it is often possible to rewrite the query instead of having to perform a nested cursor.

Should you still need to nest your cursor in a loop, your code should look something like this:

increment: LOOP
    block_cursor: BEGIN
        DECLARE cur1 CURSOR FOR SELECT person_id, publication_id  FROM p_publication WHERE person_id = new_count;
        DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;
        OPEN cur1;
        REPEAT
            FETCH cur1 INTO pub_id, per_id;
            IF NOT done THEN
                INSERT INTO test.t2 VALUES (pub_id, per_id);
            END IF;
            SET new_count = new_count + 1;
        UNTIL done END REPEAT;
        CLOSE cur1;
        IF !(new_count < old_count ) THEN
            LEAVE increment;
        END IF;
    END block_cursor;
END LOOP increment;