Join and max date with null values

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