Question

What is the best way of making the following T-SQL query valid:

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)

Gives the error

Cannot use an aggregate or a subquery
in an expression used for the group
by list of a GROUP BY clause.

Maybe someone else can answer why this is not allowed. A couple of ways which are valid

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  

and

;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