SQL Query: A constant expression was encountered in the ORDER BY list - sql

Question

I'm getting the following error:
A constant expression was encountered in the ORDER BY list, position 1.
I'm trying to order by cell values in a column. The query is three union select statements. Code below:
Declare #RefDate date = '2019-09-04';
Select
'On Roll' as 'Student Group',
sum(case when year = '7' then 1 else 0 end) as 'Y7 No.', sum(case when year = '7' then 1 else 0 end)/cast(sum(1) as decimal) as 'Y7 %',
sum(case when year = '8' then 1 else 0 end) as 'Y8 No.', sum(case when year = '8' then 1 else 0 end)/cast(sum(1) as decimal) as 'Y8 %',
sum(case when year = '9' then 1 else 0 end) as 'Y9 No.', sum(case when year = '9' then 1 else 0 end)/cast(sum(1) as decimal) as 'Y9 %',
sum(case when year = '10' then 1 else 0 end) as 'Y10 No.', sum(case when year = '10' then 1 else 0 end)/cast(sum(1) as decimal) as 'Y10 %',
sum(case when year = '11' then 1 else 0 end) as 'Y11 No.', sum(case when year = '11' then 1 else 0 end)/cast(sum(1) as decimal) as 'Y11 %',
sum(1) as 'All' from totals
where leaving is null and admission <= GETDATE() and enrolment in ('single registration','main - dual registration')
Union
Select
'New',
sum(case when year = '7' then 1 else 0 end) as 'Y7 No.', sum(case when year = '7' then 1 else 0 end)/cast(sum(1) as decimal) as 'Y7 %',
sum(case when year = '8' then 1 else 0 end) as 'Y8 No.', sum(case when year = '8' then 1 else 0 end)/cast(sum(1) as decimal) as 'Y8 %',
sum(case when year = '9' then 1 else 0 end) as 'Y9 No.', sum(case when year = '9' then 1 else 0 end)/cast(sum(1) as decimal) as 'Y9 %',
sum(case when year = '10' then 1 else 0 end) as 'Y10 No.', sum(case when year = '10' then 1 else 0 end)/cast(sum(1) as decimal) as 'Y10 %',
sum(case when year = '11' then 1 else 0 end) as 'Y11 No.', sum(case when year = '11' then 1 else 0 end)/cast(sum(1) as decimal) as 'Y11 %',
sum(1) as 'All' from totals
where admission > #RefDate and enrolment in ('single registration','main - dual registration')
Union
Select
'Leavers',
sum(case when year = '7' then 1 else 0 end) as 'Y7 No.', sum(case when year = '7' then 1 else 0 end)/cast(sum(1) as decimal) as 'Y7 %',
sum(case when year = '8' then 1 else 0 end) as 'Y8 No.', sum(case when year = '8' then 1 else 0 end)/cast(sum(1) as decimal) as 'Y8 %',
sum(case when year = '9' then 1 else 0 end) as 'Y9 No.', sum(case when year = '9' then 1 else 0 end)/cast(sum(1) as decimal) as 'Y9 %',
sum(case when year = '10' then 1 else 0 end) as 'Y10 No.', sum(case when year = '10' then 1 else 0 end)/cast(sum(1) as decimal) as 'Y10 %',
sum(case when year = '11' then 1 else 0 end) as 'Y11 No.', sum(case when year = '11' then 1 else 0 end)/cast(sum(1) as decimal) as 'Y11 %',
sum(1) as 'All' from totals
where leaving > #RefDate and enrolment in ('single registration','main - dual registration')
order by
case 'Student Group'
when 'On Roll' then 1
when 'New' then 2
when 'Leavers' then 3
end

Answer 1

If the groups were entirely separate, you could just use aggregation. Instead, you can use apply to define the groups and then aggregate:
Select v.Student_Group,
sum(case when year = '7' then 1 else 0 end) as 'Y7 No.', sum(case when year = '7' then 1 else 0 end)/cast(sum(1) as decimal) as 'Y7 %',
sum(case when year = '8' then 1 else 0 end) as 'Y8 No.', sum(case when year = '8' then 1 else 0 end)/cast(sum(1) as decimal) as 'Y8 %',
sum(case when year = '9' then 1 else 0 end) a s 'Y9 No.',
sum(case when year = '9' then 1 else 0 end)/cast(sum(1) as decimal) as 'Y9 %',
sum(case when year = '10' then 1 else 0 end) as 'Y10 No.', sum(case when year = '10' then 1 else 0 end)/cast(sum(1) as decimal) as 'Y10 %',
sum(case when year = '11' then 1 else 0 end) as 'Y11 No.', sum(case when year = '11' then 1 else 0 end)/cast(sum(1) as decimal) as 'Y11 %',
sum(1) as All
from totals t cross apply
(values (1, 'On Roll', case when leaving is null and admission <= GETDATE() then 1 end),
(2, 'New', case when admission > #RefDate then 1 end),
(3, 'Leaving', case when leaving > #RefDate then 1 end)
) v(ord, Student_Group, flag)
where enrolment in ('single registration', 'main - dual registration') and
v.flag = 1
group by v.which, v.ord
order by v.ord;

Answer 2

Resolved by using UNION ALL rather than UNION.

Answer 3

Hope this can help.
With t1 as (
Select leaving, admission, enrolment,
(case when year = '7' then 1 else 0 end) as Y7,
(case when year = '8' then 1 else 0 end) as Y8,
(case when year = '9' then 1 else 0 end) as Y9,
(case when year = '10' then 1 else 0 end) as Y10,
(case when year = '11' then 1 else 0 end) as Y11
From totals),
t2 as (
Select 1 as [Student Group], Y7, Y8, Y9, Y10, Y11
From t1
Where leaving is null and admission <= GETDATE() and enrolment in ('single registration','main - dual registration')
Union all
Select 2, Y7, Y8, Y9, Y10, Y11
From t1
Where admission > #RefDate and enrolment in ('single registration','main - dual registration')
Union all
Select 3, Y7, Y8, Y9, Y10, Y11
From t1
Where leaving > #RefDate and enrolment in ('single registration','main - dual registration'))
/* Select result */
Select case when [Student Group] = 1
then 'On Roll'
else case when [Student Group] = 2
then 'New'
else 'Leaving'
end
end as [Student Group Name],
sum(Y7) as [Y7 No.], 1.0 * sum(Y7) / count(1) as [Y7 %],
sum(Y8) as [Y8 No.], 1.0 * sum(Y8) / count(1) as [Y8 %],
sum(Y9) as [Y9 No.], 1.0 * sum(Y9) / count(1) as [Y9 %],
sum(Y10) as [Y10 No.], 1.0 * sum(Y10) / count(1) as [Y10 %],
sum(Y11) as [Y11 No.], 1.0 * sum(Y11) / count(1) as [Y11 %]
From t2
Group by [Student Group]
--Order by [Student Group]
Or
/* Select result */
Select StudentGroup.[Student Group Name],
t3.[Y7 No.], t3.[Y7 %],
t3.[Y8 No.], t3.[Y8 %],
t3.[Y9 No.], t3.[Y9 %],
t3.[Y10 No.], t3.[Y10 %],
t3.[Y11 No.], t3.[Y11 %]
From (
Select [Student Group],
sum(Y7) as [Y7 No.], 1.0 * sum(Y7) / count(1) as [Y7 %],
sum(Y8) as [Y8 No.], 1.0 * sum(Y8) / count(1) as [Y8 %],
sum(Y9) as [Y9 No.], 1.0 * sum(Y9) / count(1) as [Y9 %],
sum(Y10) as [Y10 No.], 1.0 * sum(Y10) / count(1) as [Y10 %],
sum(Y11) as [Y11 No.], 1.0 * sum(Y11) / count(1) as [Y11 %]
From t2
Group by [Student Group]
) t3
Inner join (
Select 1 as [Student Group], 'On Roll' as [Student Group Name]
Union
Select 2, 'New'
Union
Select 3, 'Leaving'
) StudentGroup
On t3.[Student Group] = StudentGroup.[Student Group]
Order by t3.[Student Group]
Cheers!