Using two different where clauses - tsql

Question

I would like to know how to use a different WHERE clause based on a CASE or IF. I'd prefer a CASE, as the rest of the statement is complex, and I don't like the idea of that complexity being in two places with only a minor difference. However, I know cases are only used for values. I've replicated a simple version of my issue below.
Essentially, I have three tables. The first contains the master information (MasterTable). The second contains a one-to-many relationship belonging to the master table (Table1). The third is a list of selectors indicating which of the records in Table1 are to be used in this instance. I want the most recent record of Table2 to drive what is selected from Table1, with precedence given to SubID over OrderNum.
MasterTable | MasterID, OtherInfo
Table1 | T1UniqueId, MasterID, SubID, Text, OrderNum
Table2 | T2UniqueId, MasterID, SubID, OrderNum, Date
SELECT MasterID, OtherInfo, SubID
FROM MasterTable
OUTER APPLY(
SELECT TOP 1 SubID FROM Table1
WHERE Table1.MasterID=MasterTable.MasterID
CASE
WHEN
(
SELECT TOP 1 SubID FROM Table2
WHERE Table2.MasterID=MasterTable.MasterID
ORDER BY Date DESC
) Is NULL
THEN Table1.OrderNum=
(
SELECT TOP 1 OrderNum
FROM Table2
WHERE Table2.MasterId=MasterTable.MasterId
ORDER BY Date DESC
)
ELSE Table1.SubId=
(
SELECT TOP 1 SubId
FROM Table2
WHERE Table2.MasterId=MasterTable.MasterId
ORDER BY Date DESC
)
END
) SubData

Answer 1

One quick rewrite of this would result in the following:
IF ((SELECT TOP 1 SubID FROM Table2 WHERE Table2.MasterID=MasterTable.MasterID ORDER BY Date DESC) IS NULL)
BEGIN
SELECT
MasterID, OtherInfo, SubID
FROM MasterTable
OUTER APPLY(
SELECT TOP 1 SubID FROM Table1
WHERE
Table1.MasterID=MasterTable.MasterID
AND Table1.OrderNum =
(
SELECT TOP 1 OrderNum
FROM Table2
WHERE Table2.MasterId=MasterTable.MasterId
ORDER BY Date DESC
)
) SubData
END
ELSE
BEGIN
SELECT
MasterID, OtherInfo, SubID
FROM MasterTable
OUTER APPLY(
SELECT TOP 1 SubID FROM Table1
WHERE
Table1.MasterID=MasterTable.MasterID
AND Table1.SubId=
(
SELECT TOP 1 SubId
FROM Table2
WHERE Table2.MasterId=MasterTable.MasterId
ORDER BY Date DESC
)
) SubData
END
But as you noted that makes it look ugly, because you now have that complexity in two places...
I guess you could also formulate it this way (untested, but this should keep your complex logic in one place):
SELECT
MasterID, OtherInfo, SubID
FROM MasterTable
OUTER APPLY(
SELECT TOP 1 SubID FROM Table1
WHERE Table1.MasterID=MasterTable.MasterID
AND
(
(
(
SELECT
TOP 1 SubID
FROM Table2
WHERE Table2.MasterID=MasterTable.MasterID
ORDER BY Date DESC
) IS NULL
AND
Table1.OrderNum =
(
SELECT TOP 1 OrderNum
FROM Table2
WHERE Table2.MasterId=MasterTable.MasterId
ORDER BY Date DESC
)
)
OR
(
Table1.SubId =
(
SELECT
TOP 1 SubId
FROM Table2
WHERE Table2.MasterId=MasterTable.MasterId
ORDER BY Date DESC
)
)
)
) SubData

Answer 2

If SubID and OrderNum in Table1 and Table2 are the same you can utilize simple query with nested select statement:
select m.MasterID, m.OtherInfo, (
select top 1 coalesce(t2.SubID, t2.OrderNum) from Table2 t2
where t2.MasterID = m.MasterID order by date desc
) as SubID
from MasterTable m;