SQL - Return fields ONLY containing met requirement - sql

Question

I am trying to do the following:
Assign a Y/N flag as Prior Approval which includes all policies that are policies with BI limit 25/50 and PD limit 25 and no other coverage.
So this code satisfies the first requirements:
SELECT DISTINCT
PolNum
FROM Results_Vehicle
WHERE ( CovCode = 'BI'
AND CovLimit = '25/50'
)
OR ( CovCode = 'PD'
AND CovLimit = '25'
)
I can't figure out how to not include records that have other coverage. So basically for each policy they can have CovCodes of UMBI, COL, REN, BI, PD, etc. But the flag should only give a Y if for that PolNum they ONLY have BI or PD as CovCode and the CovLimit defined above.
So also I'll need to put it in the Y/N format as follows, obviously this only satisfies the first part of the requirement:
SELECT CASE WHEN ( CovCode = 'BI'
AND CovLimit = '25/50'
)
OR ( CovCode = 'PD'
AND CovLimit = '25'
) THEN 'Y'
ELSE 'N'
END AS PriorApproval
FROM Results_Vehicle

Answer 1

How about this query below. The union query gives you a combined list of all the PolNum's that have that type of coverage, so then you do a not in to give you all the policy numbers that AREN'T in that list, and then you do a not in on that list, which should leave you with the PolNum's that only have the desired coverage and no other coverage.
SELECT CASE WHEN
(
(CovCode = 'BI' AND CovLimit = '25/50')
OR
(CovCode = 'PD' AND CovLimit = '25')
and PolNum not in
(
select PolNum from Results_Vehicle as r1 where PolNum not in
(
select PolNum from Results_Vehicle as r2
where CovCode = 'BI' and CovLimit = '25/50'
union
select PolNum from Results_Vehicle as r3
where CovCode = 'PD' AND CovLimit = '25'
)
)
) THEN 'Y'
ELSE 'N'
END AS PriorApproval
FROM Results_Vehicle