Max date in view on left outer join - sql

Question

Thanks to a previous question, I found out how to pull the most recent data based on a linked table. BUT, now I have a related question.
The solution that I found used row_number() and PARTITION to pull the most recent set of data. But what if there's a possibility for zero or more rows in a linked table in the view? For example, the table FollowUpDate might have 0 rows, or 1, or more. I just want the most recent FollowUpDate:
SELECT
EFD.FormId
,EFD.StatusName
,MAX(EFD.ActionDate)
,EFT.Name AS FormType
,ECOA.Account AS ChargeOffAccount
,ERC.Name AS ReasonCode
,EAC.Description AS ApprovalCode
,MAX(EFU.FollowUpDate) AS FollowUpDate
FROM (
SELECT EF.FormId, EFD.ActionDate, EFS.Name AS StatusName, EF.FormTypeId, EF.ChargeOffId, EF.ReasonCodeId, EF.ApprovalCodeId,
row_number() OVER ( PARTITION BY EF.FormId ORDER BY EFD.ActionDate DESC ) DateSortKey
FROM Extension.FormDate EFD INNER JOIN Extension.Form EF ON EFD.FormId = EF.FormId INNER JOIN Extension.FormStatus EFS ON EFD.StatusId = EFS.StatusId
) EFD
INNER JOIN Extension.FormType EFT ON EFD.FormTypeId = EFT.FormTypeId
LEFT OUTER JOIN Extension.ChargeOffAccount ECOA ON EFD.ChargeOffId = ECOA.ChargeOffId
LEFT OUTER JOIN Extension.ReasonCode ERC ON EFD.ReasonCodeId = ERC.ReasonCodeId
LEFT OUTER JOIN Extension.ApprovalCode EAC ON EFD.ApprovalCodeId = EAC.ApprovalCodeId
LEFT OUTER JOIN (Select EFU.FormId, EFU.FollowUpDate, row_number() OVER (PARTITION BY EFU.FormId ORDER BY EFU.FollowUpDate DESC) FUDateSortKey FROM Extension.FormFollowUp EFU INNER JOIN Extension.Form EF ON EFU.FormId = EF.FormId) EFU ON EFD.FormId = EFU.FormId
WHERE EFD.DateSortKey = 1
GROUP BY
EFD.FormId, EFD.ActionDate, EFD.StatusName, EFT.Name, ECOA.Account, ERC.Name, EAC.Description, EFU.FollowUpDate
ORDER BY
EFD.FormId
If I do a similar pull using row_number() and PARTITION, I get the data only if there is at least one row in FollowUpDate. Kinda defeats the purpose of a LEFT OUTER JOIN. Can anyone help me get this working?

Answer 1

I rewrote your query - you had unnecessary subselects, and used row_number() for the FUDateSortKey but didn't use the column:
SELECT t.formid,
t.statusname,
MAX(t.actiondate) 'actiondate',
t.formtype,
t.chargeoffaccount,
t.reasoncode,
t.approvalcode,
MAX(t.followupdate) 'followupdate'
FROM (
SELECT t.formid,
fs.name 'StatusName',
t.actiondate,
ft.name 'formtype',
coa.account 'ChargeOffAccount',
rc.name 'ReasonCode',
ac.description 'ApprovalCode',
ffu.followupdate,
row_number() OVER (PARTITION BY ef.formid ORDER BY t.actiondate DESC) 'DateSortKey'
FROM EXTENSION.FORMDATE t
JOIN EXTENSION.FORM ef ON ef.formid = t.formid
JOIN EXTENSION.FORMSTATUS fs ON fs.statusid = t.statusid
JOIN EXTENSION.FORMTYPE ft ON ft.formtypeid = ef.formtypeid
LEFT JOIN EXTENSION.CHARGEOFFACCOUNT coa ON coa.chargeoffid = ef.chargeoffid
LEFT JOIN EXTENSION.REASONCODE rc ON rc.reasoncodeid = ef.reasoncodeid
LEFT JOIN EXTENSION.APPROVALCODE ac ON ac.approvalcodeid = ef.approvalcodeid
LEFT JOIN EXTENSION.FORMFOLLOWUP ffu ON ffu.formid = t.formid) t
WHERE t.datesortkey = 1
GROUP BY t.formid, t.statusname, t.formtype, t.chargeoffaccount, t.reasoncode, t.approvalcode
ORDER BY t.formid
The change I made to allow for FollowUpDate was to use a LEFT JOIN onto the FORMFOLLOWUP table - you were doing an INNER JOIN, so you'd only get rows with FORMFOLLOWUP records associated.

Answer 2

It's pretty hard to guess what's going on without table definitions and sample data.
Also, this is confusing: "the table FollowUpDate might have 0 rows" and you "want the most recent FollowUpDate." (especially when there is no table named FollowUpDate) There is no "most recent FollowUpDate" if there are zero FollowUpDates.
Maybe you want
WHERE <follow up date row number> in (1,NULL)

Answer 3

I figured it out. And as usual, I need a nap. I just needed to change my subselect to something I would swear I'd tried with no success:
SELECT field1, field2
FROM Table1 t1
LEFT JOIN (
SELECT field3, max(dateColumn)
FROM Table2
GROUP BY
field3
) t2
ON (t1.field1 = t2.field3)