mysql stored procedure question

SET @balance := 0;
SET @high := 0;
SELECT @high := GREATEST(@balance := @balance+credit-debit, @high) FROM mytable;
SELECT @high;

CREATE FUNCTION high_bal() RETURNS DECIMAL
BEGIN
  SET @balance := 0;
  SET @high := 0;

  SELECT MAX(@high := GREATEST(@balance := @balance+debit_acc-credit_acc, @high))
  INTO @high
  FROM credit_acc where credit_used_acc=63395;

  RETURN @high;
END$$

BEGIN
SET @balance := 0;
SET @high := 0;
SELECT @high := GREATEST(@balance := @balance+debit_acc-credit_acc, @high) FROM credit_acc where credit_used_acc=63395;

RETURN @high;
END$$

print("code sample");

DELIMITER ;

select max(debit-credit) from yourtable