有效地包含不在SQL查询的Group By中的列

Question

特定

表A.

Id   INTEGER
Name VARCHAR(50)

表B.

Id   INTEGER
FkId INTEGER  ; Foreign key to Table A

我希望计算每个FkId值的出现次数:

SELECT FkId, COUNT(FkId) 
FROM B 
GROUP BY FkId

现在我只想输出名称Table A.

这不起作用:

SELECT FkId, COUNT(FkId), a.Name
FROM B b
INNER JOIN A a ON a.Id=b.FkId
GROUP BY FkId

因为a.Name未包含在GROUP BY子句中(产生is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause错误).

重点是从这样的输出转移

FkId  Count
1      42
2      25

输出像这样

FkId  Count  Name
1      42     Ronald
2      22     John

上有SO该错误消息不少比赛,但有些如/sf/answers/451986111/有意见,如"将产生在桌子上3次扫描,而不是1,所以不会结垢".

如何在查询输出中有效地包含联接Table B(与...具有1:1关系FkId)的字段？

Answer 1

你可以尝试这样的事情:

   ;WITH GroupedData AS
   (
       SELECT FkId, COUNT(FkId) As FkCount
       FROM B 
       GROUP BY FkId
   ) 
   SELECT gd.*, a.Name
   FROM GroupedData gd
   INNER JOIN dbo.A ON gd.FkId = A.FkId

创建一个CTE(公用表表达式)来处理对您的分组/计数Table B,然后将该结果(每行一行FkId)连接到Table A并将更多列从Table A您的最终结果集中获取.