Question

SELECT 
   count(ProductID) as CountAllProducts,
   count(CASE WHEN PriceType = '1' THEN ProductID ELSE 0 END) as CountPriceType1,
   count(CASE WHEN PriceType = '2' THEN ProductID ELSE 0 END) as CountPriceType2
FROM Products

我们得不到正确的结果：

CountAllProducts   CountPriceType1   CountPriceType2
     10                 10                 10

但应该是：

CountAllProducts   CountPriceType1   CountPriceType2
     10                 10                 0

请告诉我哪里有错误？

Answer 1

对于参数为非null的任何行，COUNT（）计数为1.

换句话说，零的数量与1的数量或香蕉或其他任何数量相同。

你可能想要这个：

SELECT 
   count(ProductID) as CountAllProducts,
   count(CASE WHEN PriceType = '1' THEN ProductID ELSE NULL END) as CountPriceType1,
   count(CASE WHEN PriceType = '2' THEN ProductID ELSE NULL END) as CountPriceType2
FROM Products

您可以稍微缩短一点，因为CASE如果值不匹配则隐式返回NULL。

SELECT 
   count(ProductID) as CountAllProducts,
   count(CASE WHEN PriceType = '1' THEN ProductID END) as CountPriceType1,
   count(CASE WHEN PriceType = '2' THEN ProductID END) as CountPriceType2
FROM Products

另一种方法是使用SUM（）而不是COUNT（），并确保SUM（）的参数是1（计算它）或0（不计算它）。因为一堆1和0的总和等于1的计数。

SELECT 
   count(ProductID) as CountAllProducts,
   SUM(CASE WHEN PriceType = '1' THEN 1 ELSE 0 END) as CountPriceType1,
   SUM(CASE WHEN PriceType = '2' THEN 1 ELSE 0 END) as CountPriceType2
FROM Products

Answer 2

count计算非空值，因此即使计算0

SELECT 
   count(ProductID) as CountAllProducts,
   count(CASE WHEN PriceType = '1' THEN ProductID ELSE NULL END) as CountPriceType1,
   count(CASE WHEN PriceType = '2' THEN ProductID ELSE NULL END) as CountPriceType2
FROM Products

或

SELECT 
   count(ProductID) as CountAllProducts,
   sum(PriceType = '1') as CountPriceType1,
   sum(PriceType = '2') as CountPriceType2
FROM Products

多重错误在哪里？

2 个答案: