WHERE Clause does not accept just defined column - sql

Question

This Code does not work:
SELECT
(
SELECT [T_Licence].[isInstalled]
FROM [T_Licence]
WHERE [T_Licence].[System] = [T_System].[ID]
AND [T_Licence].[Software] = 750
) AS [IsInstalled] ,*
FROM [T_System]
WHERE [IsInstalled] = 1
I have to do it this way, but this makes the whole code so complicated. I really dont want that:
SELECT
(
SELECT [T_Licence].[isInstalled]
FROM [wf_subj_all].[T_Licence]
WHERE [T_Licence].[System] = [T_System].[ID]
AND [T_Licence].[Software] = 750
) AS [IsInstalled] ,*
FROM [wf_subj_it].[T_System]
WHERE
(
SELECT
(
SELECT [T_Licence].[isInstalled]
FROM [wf_subj_all].[T_Licence]
WHERE [T_Licence].[System] = [T_System].[ID]
AND [T_Licence].[Software] = 750
)
) = 1
Is there any way to do it like shown in the first code snippet?
So that the code stays somehow readeble.
thx very much

Answer 1

Try this one -
SELECT *
FROM wf_subj_it.T_System s
CROSS APPLY (
SELECT /*TOP(1)*/ t.isInstalled
FROM wf_subj_all.T_Licence t
WHERE t.[System] = s.ID
AND t.Software = 750
) t
WHERE t.isInstalled = 1

Answer 2

Just wrap the query with an outer select and it should work.
SELECT *
FROM
(
SELECT
(
SELECT [T_Licence].[isInstalled]
FROM [T_Licence]
WHERE [T_Licence].[System] = [T_System].[ID]
AND [T_Licence].[Software] = 750
) AS [IsInstalled], *
FROM [T_System]
) As tbl1
WHERE [IsInstalled] = 1

Answer 3

The assumptions I made for my answer:
You're trying to select the systems (table T_System) which have software with id=750 installed
The table T_License contains the installed information
There is a 1:n relation between T_System and T_License: T_License may contain 0, 1 or more records per Sytem value...
but the combination System plus Software is unique
I think this will work
SELECT l.[isInstalled], s.*
FROM [wf_subj_it].[T_System] AS s
INNER JOIN [wf_subj_it].[T_License] AS l
ON l.[System] = s.[ID]
AND l.[Software] = 750
AND l.isInstalled = 1