Order by calculated column with alias inside case expression - sql

Question

I've got a problem with my order by clause when using a calculated column with an alias as below:
This order by works without any problem
declare #Mode int = 1
declare #Sort nvarchar(max) = 'engname'
select top 10 School.Id as EntityId,
School.EnglishName as EntityEnglishName,
School.Name as EntityNativeName,
case
when #Mode = 0 then 0
when #Mode = 1 then 1
when #Mode = 2 then 2
end as ActiveStudents
from V_SchoolMinimized as School
Order By ActiveStudents
The following query has an error:
Invalid column name 'ActiveStudents'
declare #Mode int = 1
declare #Sort nvarchar(max) = 'engname'
select top 10 School.Id as EntityId,
School.EnglishName as EntityEnglishName,
School.Name as EntityNativeName,
case
when #Mode = 0 then 0
when #Mode = 1 then 1
when #Mode = 2 then 2
end as ActiveStudents
from V_SchoolMinimized as School
Order By
case when #Sort is null then School.Id end,
case when #Sort = 'engname' then ActiveStudents end
How can I use ActiveStudents within the conditional order by clause as shown?

Answer 1

So while you can use a calculated column in your ORDER BY clause (but not in other clauses such as GROUP BY), you cannot then apply further calculations or conditions - it must be used exactly as created.
There are a whole bunch of ways to solve this problem. Which approach you use will come down to some combination of:
Which option is clearer to you as the developer
Which option performs better
Which option fits into your existing query better
Option 1: Repeat the logic
I don't recommend this option because it violates the DRY principle thereby making it harder to maintain and easier to make mistakes.
select top 10
S.Id as EntityId
, S.EnglishName as EntityEnglishName
, S.[Name] as EntityNativeName
, case
when #Mode = 0 then 0
when #Mode = 1 then 1
when #Mode = 2 then 2
end as ActiveStudents
from V_SchoolMinimized as S
order by
case when #Sort is null then S.Id end
, case when #Sort = 'engname' then
case
when #Mode = 0 then 0
when #Mode = 1 then 1
when #Mode = 2 then 2
end
end;
The rest of the options are sub-query variations the choice of which comes down to the comments provided as the start.
Option 2: Use a derived table sub-query
select top 10
S.Id as EntityId
, S.EnglishName as EntityEnglishName
, S.[Name] as EntityNativeName
, S.ActiveStudents
from (
select *
, case
when #Mode = 0 then 0
when #Mode = 1 then 1
when #Mode = 2 then 2
end as ActiveStudents
from V_SchoolMinimized
) as S
order by
case when #Sort is null then S.Id end
, case when #Sort = 'engname' then S.ActiveStudents end;
Option 3: Use a CTE (Common Table Expression)
with cte as (
select *
, case
when #Mode = 0 then 0
when #Mode = 1 then 1
when #Mode = 2 then 2
end as ActiveStudents
from V_SchoolMinimized
)
select top 10
S.Id as EntityId
, S.EnglishName as EntityEnglishName
, S.[Name] as EntityNativeName
, S.ActiveStudents
from cte
order by
case when #Sort is null then S.Id end
, case when #Sort = 'engname' then S.ActiveStudents end;
Option 4: Use CROSS APPLY
select top 10
S.Id as EntityId
, S.EnglishName as EntityEnglishName
, S.[Name] as EntityNativeName
, A.Students
from V_SchoolMinimized as S
cross apply (
values (
case
when #Mode = 0 then 0
when #Mode = 1 then 1
when #Mode = 2 then 2
end
)
) as A (Students)
order by
case when #Sort is null then S.Id end
, case when #Sort = 'engname' then A.Students end;
Note: I suggest keeping your table aliases nice and short, 1-2 characters where possible, occasionally 3.