I am stuck on getting a previous value - sql

Question

I have been working on this SQL code for a bit and I cannot get it to display like I want. I have an operation that we send parts outside of our business but there is no time stamp on when that operation sent out.
I am taking the previous operation's last labor date and the purchase order creation date to try and find out how long it takes that department to issued a purchase order.
I have tried LAST_Value to add to my query. I have even played with LAG and couldn't get a anything but errors.
SELECT
JobOpDtl.JobNum,
JobOpDtl.OprSeq,
JobOpDtl.OpDtlDesc,
LastValue.ClockInDate,
LastValue.LastValue
FROM Erp.JobOpDtl
LEFT OUTER JOIN Erp.LaborDtl ON
LaborDtl.JobNum = JobOpDtl.JobNum
and LaborDtl.OprSeq = JobOpDtl.OprSeq
LEFT OUTER JOIN (
Select
LaborDtl.JobNum,
LaborDtl.OprSeq,
MAX(LaborDtl.ClockInDate) as ClockInDate,
LAST_VALUE (LaborDtl.ClockInDate) OVER (PARTITION BY OprSeq ORDER BY JobNum) as LastValue
FROM Erp.LaborDtl
GROUP BY
LaborDtl.JobNum,
LaborDtl.OprSeq,
LaborDtl.ClockInDate
) as LastValue ON
JobOpDtl.JobNum = LastValue.JobNum
and JobOpDtl.OprSeq = LastValue.OprSeq
WHERE JobOpDtl.JobNum = 'PA8906'
GROUP BY
JobOpDtl.JobNum,
LastValue.OprSeq,
JobOpDtl.OpDtlDesc,
JobOpDtl.OprSeq,
LastValue.ClockInDate,
LastValue.LastValue
No errors, just not displaying how I am wanting it.
I would like it to display the OperSeq with the previous OperSeq last transaction date.

Answer 1

The basic function you want is LAG (as you suggested) but you need to wrap it in a COALESCE. Here is a sample code that illustrates the concept
SELECT * INTO #Jobs
FROM (VALUES ('P1','Step1', '2019-04-01'), ('P1','Step2', '2019-04-02')
, ('P1','Step3', '2019-04-03'), ('P1','Step4', NULL),
('P2','Step1', '2019-04-01'), ('P2','Step2', '2019-04-03')
, ('P2','Step3', '2019-04-06'), ('P2','Step4', NULL)
) as JobDet(JobNum, Descript, LastDate)
SELECT *
, COALESCE( LastDate, LAG(LastDate,1)
OVER(PARTITION BY JobNum
ORDER BY COALESCE(LastDate,GETDATE()))) as LastValue
FROM #Jobs
ORDER BY JobNum, Descript
DROP TABLE #Jobs
To apply it to your specific problem, I'd suggest using a COMMON TABLE EXPRESSION that replaces LastValue and using that instead of the raw table for your queries.
Your example picture doesn't match any tables you reference in your code (it would help us significantly if you included code that created temp tables matching those referenced in your code) so this is a guess, but it will be something like this:
;WITH cteJob as (
SELECT JobNum, OprSeq, OpDtlDesc, ClockInDate
, COALESCE( LastValue, LAG(LastValue,1)
OVER(PARTITION BY JobNum
ORDER BY COALESCE(LastValue,GETDATE()))) as LastValue
FROM Erp.JobOptDtl
) SELECT *
FROM cteJob as J
LEFT OUTER JOIN LaborDtl as L
on J.JobNum = JobNum
AND J.OprSeq = L.OprSeq
BTW, if you clean up your question to provide a better example of your data (i.e. SELECT INTO sttements like in the start of my answer that produce tables that correspond to the tables in your code instead of an image of an excel file) I might be able to get you closer to what you need, but hopefully this is enough to get you on the right track and it's the best I can do with what you've provided so far.