问我应该在哪里编写MySQL触发器

EN

Create Trigger sales_bi_trg
BEFORE INSERT ON sales
FOR EACH ROW
BEGIN
DECLARE num_row INTEGER ;
DECLARE tot_rows INTEGER ;
SELECT COUNT(*)
INTO tot_rows
FROM sales
WHERE employee_id = NEW.employee_id ;
   IF num_row > 0 THEN
       UPDATE perfomance 
    SET total_sales = NEW.sale_amt + total_sales,
    ave_sale = total_sales/(tot_rows + 1)
    WHERE employee_id = NEW.employee_id ;
   ELSE
   INSERT INTO perfomance
(employee_id, name, total_sales,ave_sale)
VALUES (NEW.employee_id, NEW.name, NEW.sale_amt, NEW.sale_amt) ;
   END IF ;