Question

这更像是理论问题而不是问题。假设我有一个包含三列id, Country1, Country2的表格。

我想运行简单的示例查询：

SELECT 
     CASE WHEN Country1 = '' THEN Country2 ELSE Country1 END AS [Country], 
     COUNT(id) AS [Countries_count]
FROM ExampleTable
GROUP BY 
     CASE WHEN Country1 = '' THEN Country2 ELSE Country1 END

无论我将整个非聚合表达式

，我的查询都会有效

GROUP BY 
     CASE WHEN Country1 = '' THEN Country2 ELSE Country1 END

或仅包含此表达式中的列

GROUP BY 
     Country1,
     Country2

在GROUP BY子句中。

我是否应该将GROUP BY子句放入每个非聚合表达式（有时真的很大）或者我只能放置这些表达式中的列？如果我能/不能，为什么？

Answer 1

您在询问哪个更适合查询：

GROUP BY CASE WHEN Country1 = '' THEN Country2 ELSE Country1 END

或

GROUP BY Country1, Country2

答案通常是第一个。但是，这取决于你想做什么。

考虑以下数据：

ID       Country1       Country2
 1                             A
 2              A              A
 3              A

此查询：

SELECT (CASE WHEN Country1 = '' THEN Country2 ELSE Country1 END) AS [Country], 
       COUNT(id) AS [Countries_count]
FROM ExampleTable
GROUP BY (CASE WHEN Country1 = '' THEN Country2 ELSE Country1 END)

返回一行总计数

A    3

此版本

SELECT (CASE WHEN Country1 = '' THEN Country2 ELSE Country1 END) AS [Country], 
       COUNT(id) AS [Countries_count]
FROM ExampleTable
GROUP BY Country1, Country2

返回三行：

A    1
A    1
A    1

在大多数情况下，我想要一行计数为3.在某些情况下，这可能是所需的输出。

GROUP BY子句中的SQL表达式或列

1 个答案: