Mysql觸發器在2個表中進行計算？

Question

我試圖做一個觸發器必須做一個減法，如果order_status從0更改爲1或2.更新的order_status行中的數量值應從另一個表中的quantity_in_stock中減去。這是我試過的，但它不適合我。

begin 
    DECLARE orderQuantity INT(11); 
    if old.order_status = 0 and new.order_status != 0 then 
    select quantity into orderQuantity from `order` where id=new.id; 
    update product_in_stock 
    set quantity_in_stock = sum(quantity_in_stock - orderQuantity) 
    where id=1; 
    end if; 
end 

Answer 1

除非你自己定義sum功能，您使用的方法是錯誤的

，而不是set quantity_in_stock = sum(quantity_in_stock - orderQuantity)

應該set quantity_in_stock = sum(quantity_in_stock) - sum(orderQuantity)

但同樣，你不能使用直接像這樣的聚合函數，除非它在having子句中。

你能做什麼，declare two variable -> fetch the sum separately and store them to variable -> use them像

DECLARE sum_quantity_in_stock INT(11); 
DECLARE sum_orderQuantity INT(11); 

select sum(quantity_in_stock) into sum_quantity_in_stock from sometable; 
select sum(orderQuantity) into sum_orderQuantity from sometable; 


set quantity_in_stock = sum_quantity_in_stock - sum_orderQuantity