How does HANDLER FOR NOT FOUND work and what is its usage?

Question

Still kinda new to MySQL. I was wondering what is HANDLER FOR NOT FOUND, how does it work and most importantly: what is its usage?

Answer 1

A HANDLER is for catching exceptions.

When reading from a cursor, reading past the end of the cursor throws a NOT FOUND exception, rather than returning an endless stream of NULL, so you have to catch this exception.

DECLARE val1 INT DEFAULT NULL;
DECLARE done TINYINT DEFAULT FALSE;

DECLARE c1 CURSOR FOR SELECT id FROM t1;

-- when the NOT FOUND condition fires, "done" -- which defaults to FALSE -- will be set to true,
-- and since this is a CONTINUE handler, execution continues with the next statement.   

DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;

OPEN c1;

my_loop: 
LOOP

  FETCH NEXT FROM c1 INTO val1; 
  IF done THEN -- this will be true when we are out of rows to read, so we go to the statement after END LOOP.
    LEAVE my_loop; 
  ELSE
    -- maybe do more stuff here
  END IF;
END LOOP;

-- procedure continues here...

Copied in part from my example here.