mysql - Trigger can't work properly when check condition -

i want store records in 2 way.

1. if exists, update information
2. if dosn't exist, insert new record

here outline schemas tables:

product_purchase_item has product_purchase_item_id, product_id, quantity columns
product_stock has product_stock_id, product_id, product_total_quantity columns

i creating trigger

 delimiter //  create trigger store_check after insert on product_purchase_item  each row  begin   declare x integer;   set x =(select product_id product_stock product_id = new.product_id);   if new.product_id !=x     insert product_stock values(null,new.product_id,new.quantity);   else    update product_stock     set product_total_quantity=product_total_quantity+new.quantity    product_id=new.product_id;   end if;  end;//  delimiter;// 

the problem in product_stock table, insert query doesn't work when product record not exist, update query work when product record exist.

is insert statement "doesn't work"? or, problem insert statement not being executed @ all?

what happens when preceding select query not return row? value gets assigned x?

when x has value, conditional test "foo != x" return true, or return else (like false, or null)?

have tried this?

if new.product_id = x    update ... else    insert ...     end if; 

(i know it's bad practice answer question question; seemed apropros, since answering answer question wanted ask.)