MySQL 使用 sum 加速子查询

Question

我需要一个查询来获取实际库存的概览（每个唯一产品 ID 只有一行）以及所有销售和采购订单。现在，我们使用两个子查询（销售订单和采购订单），因为可能有多个销售订单和采购订单。 由于有两个子查询，此查询非常慢。是否有另一种方法来获取销售订单的总和和采购订单的总和，例如常规左连接？

当前查询

SELECT stock.quanity, salesOrders.quantity, purchaseOrders.quantity
FROM stock
LEFT JOIN (
   SELECT product_id, sum(quantity) as quantity
   FROM sales_orders
   GROUP BY product_id
) salesOrders ON stock.product_id=salesOrders.product_id
LEFT JOIN (
   SELECT product_id, sum(quantity) as quantity
   FROM purcahse_orders
   GROUP BY product_id
) purchaseOrders ON stock.product_id=purchaseOrders.product_id

Answer 1

如果索引正确，相关子查询可能会更快：

SELECT s.quantity, 
       (SELECT sum(so.quantity) FROM sales_orders so WHERE s.product_id = so.product_id
       ) as sales_quantity ,
       (SELECT sum(po.quantity) FROM purchase_orders po WHERE s.product_id = po.product_id
       ) as purchase_quantity 
FROM stock s ;

为了希望提高性能，您需要在sales_orders(product_id, quantity) 和purchase_orders(product_id, quantity) 上建立索引。