Question

鉴于

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

现在我只想从Table A输出名称。

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子句中未包含GROUP BY（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移动

FkId  Count
1      42
2      25

像这样输出

FkId  Count  Name
1      42     Ronald
2      22     John

对于该错误消息，SO上有很多匹配，但有些例如https://stackoverflow.com/a/6456944/141172有一些评论，例如“将在桌面上生成3次扫描，而非1，因此无法扩展”。

如何有效地在查询输出中包含加入的Table B（与FkId的关系为1：1）中的字段？

Answer 1

您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Table B的分组/计数，然后将该结果（每FkId一行）加入Table A并获取更多内容Table A中的列到您的最终结果集中。

Answer 2

您是否尝试将字段添加到论坛？

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

Answer 3

select t3.Name, t3.FkId, t3.countedFkId from (a t1 
  join (select t2.FkId, count(FkId) as countedFkId from b t2 group by t2.FkId) 
  on t1.Id = t2.FkId) t3;

有效地包含不在SQL查询的Group By中的列

3 个答案: