集團由一個子查詢

Question

什麼讓下面的T-SQL查詢有效的最佳途徑：

select 
    count(*), 
    (... a subquery that returns one result ...) as [Bar] 
from Foo foo 
group by [Bar] 

Answer 1

SELECT COUNT(*), 
     (SELECT TOP 1 name 
     FROM sys.objects 
     ORDER BY object_id%number) name 
FROM master..spt_values 
WHERE number > 0 
GROUP BY (SELECT TOP 1 name 
      FROM sys.objects 
      ORDER BY object_id%number) 

給出了錯誤

不能使用聚合或子查詢 GROUP BY子句列表中用於組 的表達式。

也許別人可以回答爲什麼這是不允許的。一對夫婦的方式既有效

SELECT COUNT(*), 
     oa.name 
FROM master..spt_values 
     OUTER APPLY (SELECT TOP 1 name 
        from sys.objects 
        ORDER BY object_id%number) oa 
WHERE number > 0 
GROUP BY oa.name 

和

;WITH T AS 
(
SELECT number, 
     (SELECT TOP 1 name 
     from sys.objects 
     ORDER BY object_id%number) name 
FROM master..spt_values 
WHERE number > 0 
) 
SELECT COUNT(*), 
     name 
FROM T 
GROUP BY name