Question

我试图改进以下CASE语句，只计算一次差异。我这样做是为了避免负数：

 SELECT (CASE WHEN ((SELECT 100 - (SELECT COUNT(CustomerId) FROM Customers)) > 0) 
         THEN       (SELECT 100 - (SELECT COUNT(CustomerId) FROM Customers)) 
         ELSE       (0) 
         END)

这不仅看起来很愚蠢，而且也不是线程安全的。我尝试了以下操作，但是收到错误消息“无效的列名'diff'。”

 SELECT (CASE WHEN ((SELECT 100 - (SELECT COUNT(CustomerId) FROM Customers) as diff) > 0) 
         THEN  (diff) 
         ELSE  (0) 
         END)

如何简化？是否有内置的SQL函数已经完成这项工作？

编辑：抱歉，忘记提及select语句在视图声明中，所以我无法声明变量。

Answer 1

DECLARE @diff int
SELECT @diff = 100 - COUNT(*) FROM Customers

SELECT CASE WHEN @diff > 0 THEN diff           
            ELSE 0           
       END as Diff

Answer 2

如果我遵循你的逻辑，那就应该这样做：

SELECT COALESCE ((SELECT 100 - COUNT(CustomerID)
FROM Customers
HAVING COUNT(CustomerID) < 100), 0)

略微更优雅。

如果有基于行的MAX（）函数，我们也可以使用它。当然，你可以轻松地写一个。

Answer 3

您可以使用变量来启动查询结果，这样您就不必执行两次。例如：

DECLARE @CustomerCount INT
SELECT @CustomerCount = COUNT(CustomerId) FROM Customers
SELECT CASE WHEN (100 - @CustomerCount > 0) 
     THEN 100 - @CustomerCount
     ELSE (0) 
     END

Answer 4

你是对的，你只想算一次，你想要这样做，这样就可以清楚地表明你的逻辑是什么。

SELECT CASE WHEN cnt > 100 THEN 0
       ELSE 100 - cnt END AS diff
FROM (SELECT COUNT(1) AS cnt
      FROM Customers) AS CustomerCnt

在SQL语句中简化CASE

4 个答案: