Join and max date with null values - sql

Question

I need to return the most recent (max)date for a patient_ID where a vital value exists - it's not simply the max. encounter date for the patient_ID, it must have a corresponding vital value. I also only want encounters where vitals value <>'' and Date >= '2020-01-01' and vital_ID = 232268.
Encounters (enc)
Patient_ID
Encounter_ID
Date
1
11
1/4/2020
1
12
1/15/2020
1
13
3/6/2020
2
14
1/12/2020
3
15
3/16/2020
3
16
4/19/2020
4
17
6/2/2020
4
18
6/12/2020
4
19
9/1/2020
Vitals
Encounter_ID
Vital_ID
Value
11
232268
4.8
12
232268
4.6
14
232268
3.1
16
232268
3.2
17
232268
4.1
18
232268
4.7
Desired Outcome
Patient_ID
Encounter_ID
Date
Value
1
12
3/6/2020
4.6
2
14
1/12/2020
3.1
3
16
4/19/2020
3.2
4
18
9/1/2020
4.7
I tried this, but it returned only the vitals_encounter_ID IF it = max(date) of the encounter for the patient_ID (so did not include patient_ID if vitals were not taken on the max(date) - for instance, it negates patient_ID 1 all together because vitals weren't taken on encounter_ID 13:
select v.encounterID, e.patientID, e.date, v.value, v.vitalID
from vitals v
left join enc e on
e.encounterID = v.encounterID and
v.vitalID = 232268 and
v.value <> '' and
e.date = (select max(date) from enc where patientID=e.patientID)
where e.date >= '2020-01-01'
Cognos 8. I'm new, so please don't eat me alive...

Answer 1

If I follow you correctly, you need the two tables in the correlated subquery:
select v.encounterid, e.patientid, e.date, v.value, v.vitalid
from enc e
inner join vitals v on v.encounterid = e.encounterid
where
v.vitalid = 232268
and v.value <>''
and e.date = (
select max(e1.date)
from enc e1
inner join vitals v1 on v1.encounterid = e1.encounterid
where
e1.patientid = e.patientid
and v1.vitalid = v.vitalid
and v1.value <> ''
and e.date >= '2020-01-01'
)
I don't know if Cognos supports window functions. But if it does, the query can be much simpler phrased:
select *
from (
select v.encounterid, e.patientid, e.date, v.value, v.vitalid,
row_number() over(partition by e.patientid order by e.date)
from enc e
inner join vitals v on v.encounterid = e.encounterid
where v.vitalid = 232268 and v.value <> ''
) t
where rn = 1