Case Statement as vals Outer Apply - sql

Question

I am trying to add a computed column to a table. Currently I have the following CASE statement building a calculated column in a SELECT statement,
--but I want to use this column to determine subsequent columns (sooo.. adding a computed column is somewhat a workaround to avoid complex CTE that i'm not sure i can figure out - let me know if CTE or functions is a better way to go)
--- I want the CASE statement to be a computed column in the table itself
SELECT top 1000
L.[MsgDate]
,C.[AlarmType]
,L.[Type]
,L.[UsrStr1]
,L.[UsrStr4]
,L.[UsrStr5]
,L.[UsrStr12]
,L.[UsrStr15]
,CASE
WHEN EXISTS
(
SELECT *
FROM Breinigsville.dbo.SCADA_SR S
WHERE S.SCADA_SR_TAG = L.UsrStr15 and
((L.[UsrStr4]='analog' and C.[AlarmType] like '%HH%') or (L.[UsrStr4]='status'))
)
THEN 1
ELSE 0
END AS [Safety]
FROM [Breinigsville].[dbo].[LMFW] L
full outer join [Breinigsville].[dbo].[_AcknowledgedAlarms] C
on
L.SeqNo=C.SeqNo2
WHERE (
L.[Type]='Ack' AND
L.UsrStr12 LIKE '%CCXOS%' AND
L.UsrStr12 NOT LIKE '%CCXOS5' AND
L.UsrStr12 NOT LIKE '%CCXOS6' AND
L.UsrStr12 NOT LIKE '%CCXOS9' AND
L.UsrStr12 NOT LIKE '%CCXOS12' AND
L.MsgDate>getdate()-1
)
order by L.SeqNo desc

Answer 1

Use outer apply:
FROM [Breinigsville].[dbo].[LMFW] L full outer join
[Breinigsville].[dbo].[_AcknowledgedAlarms] C
on L.SeqNo = C.SeqNo2 OUTER APPLY
(SELECT (CASE WHEN COUNT(*) > 0 THEN 1 ELSE 0 END) as Safety
FROM Breinigsville.dbo.SCADA_SR S
WHERE S.SCADA_SR_TAG = L.UsrStr15 and
((L.[UsrStr4] = 'analog' and C.[AlarmType] like '%HH%') or
(L.[UsrStr4] = 'status')
)
) vals
Then you can use vals.Safety anywhere in the query.
Note: this version uses count(*). If performance is at all a concern, then you can get what you want using an additional subquery. I answered this way, because the structure of the query seems easier to follow.
Incidentally, you cannot easily put a subquery in as a computed column. You can do it, but it requires creating a user-defined function, and using that for the computed column. OUTER APPLY is definitely simpler, and you can even put this logic into a table-valued function or view.