使用连接信息进行计数

Question

我有两张桌子。

一张表记录orderstock，有FK_stock和FK_orderNo

我想计算每件库存商品的订单数量。以下代码可以正常工作：

(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，我希望对此进行补充，以便仅列出不易腐烂 (stock.perishable=0) 的库存物品。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？当我尝试将库存表中的信息组合到此目的时，每项库存都是单独计算的。

(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）：

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

Answer 1

我猜你在找conditional count

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

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。像这样的

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