sql - SQL Server：在聚合函数中使用用户定义的列

Question

例如，我有

SELECT SUBSTRING([Path], 0, 5) AS [Directory]
      , COUNT([Directory])
FROM [MyDatabase]
GROUP BY [Directory];

但我收到“无效的列名”错误。

Answer 1

这应该有效：

SELECT SUBSTRING([Path], 0, 5) AS [Directory]
      , COUNT(1)
FROM [MyDatabase]
GROUP BY SUBSTRING([Path], 0, 5);

Answer 2

问题是 count() 中使用的列名中的别名，我也不明白为什么在计算时需要两列。尝试

SELECT COUNT(SUBSTRING([Path], 0, 5))      
    FROM [MyDatabase];

如果你需要两列，那么在这里：

   SELECT SUBSTRING([Path], 0, 5) AS [Directory]
  , COUNT(SUBSTRING([Path], 0, 5) )
   FROM [MyDatabase]
   GROUP BY SUBSTRING([Path], 0, 5);