mysql - 使用连接信息计数

Question

我有两个表。

一张表记录了订单库存，有FK_stock和FK_orderNo

我想统计每个库存商品的订单数量。以下代码可以正确地执行此操作:

(1)
    SELECT orderstock.FK_orderNo, Count(orderstock.FK_stock) AS CountOfFK_stock
     FROM stock INNER JOIN orderstock ON stock.StockID = orderstock.FK_stock 
    GROUP BY orderdemo.FK_orderNo 

但是，我想补充一点，只列出不易腐烂的库存元素 (stock.perishable=0)。所以像

SELECT orderstock.FK_orderNo, Count(orderstock.FK_stock) AS CountOfFK_stock 
FROM stock INNER JOIN orderstock ON stock.stockID = orderstock.FK_stock 
WHERE stock.perishable=0 
GROUP BY orderstock.FK_orderNo 

我如何访问与 FK_stock 相关的信息来完成这项工作？为此，当我尝试合并库存表中的信息时，每项库存都单独计算。

(1) 的结果

 FK_OrderNo     CountOfFK_Stock     
 9      10
104     8
105     3
106     10
107     8
108     10
109     11
110     9

期望的结果(类似的东西):

 FK_OrderNo     CountOfFK_Stock     
 9      7
104     8
105     3
106     4
107     7
108     2
109     11
110     6 

Answer 1

我猜你正在寻找条件计数

将 where 子句过滤器移动到 Count Aggregate 并使计数聚合仅在 stock.perishable = 0 时计算记录。

SELECT orderdemo.fk_orderno, 
       Count(CASE 
               WHEN stock.perishable = 0 THEN 1 
             END) AS nonperishable_count 
FROM   stock 
       INNER JOIN orderdemo 
               ON stock.studentid = orderdemo.fk_stock 
GROUP  BY orderdemo.fk_orderno 

Count 聚合也可以替换为 SUM 聚合。像这样

   Sum(CASE 
         WHEN stock.perishable = 0 THEN 1 
         ELSE 0 
       END) AS nonperishable_count