Question

I have this query :

SELECT pih.PID, piv.TSV, piv.Variable_ID, piv.Value_ID, piv.Manual_Value
FROM Process_Instance_Value AS piv
INNER JOIN Process_Instance_History AS pih ON piv.PID = pih.PID
WHERE piv.Variable_ID IN ( SELECT Variable_ID 
                           FROM someTable 
                           WHERE Transition_ID = @someValue) 
AND piv.PID = @someValue

I get this :

My problem is that those rows are duplicate for the use I want them at this point.
Is it possible to only get rows on DISTINCT TSV? Or something like that?

Thank you

Answer 1

You can do this:

;WITH CTE
AS
(
    SELECT 
      pih.PID, 
      piv.TSV, 
      piv.Variable_ID, 
      piv.Value_ID, 
      piv.Manual_Value,
      ROW_NUMBER() OVER (PARTITION BY piv.TSV ORDER BY pih.PID) rownum
    FROM Process_Instance_Value AS piv
    INNER JOIN Process_Instance_History AS pih ON piv.PID = pih.PID
    WHERE piv.Variable_ID IN ( SELECT Variable_ID 
                               FROM someTable 
                               WHERE Transition_ID = @someValue) 
    AND piv.PID = @someValue
)
SELECT *
FROM CTE
WHERE rownum = 1;

Note that: This will return the minimum pih.PID for each distinct piv.TSV if you want the max, just use ORDER BY pih.PID DESC instead.

How does this work:

The ranking function ROW_NUMBER() OVER() will give a ranking number for each group that has the same column listed in the PARTITION BY, in our case piv.TSV, this ranking number will be ordered, for each group, according to the ORDER BY clause defined inside the OVER clause, in our case ORDER BY pih.PID. I used a CTE instead of using subquery.

Then the I used WHERE rownum = 1, since the ranking number rownum was given for each group of records that has the same piv.TSV ranked in descendent order, then the record with rownum = 1 will be the record with the minimum pih.PID(think about it).

You can however, get the same result using an inner join with a GROUP BY like so:

    SELECT 
      pih.PID, 
      piv.TSV, 
      piv.Variable_ID, 
      piv.Value_ID, 
      piv.Manual_Value
    FROM Process_Instance_Value AS piv
    INNER JOIN
    (
        SELECT
          TSV, MIN(PID) MinPID
        FROM Process_Instance_Value 
        GROUP BY TSV
    ) piv2 ON piv.PID = piv2.MinPID
    INNER JOIN Process_Instance_History AS pih ON piv.PID = pih.PID
    WHERE piv.Variable_ID IN ( SELECT Variable_ID 
                               FROM someTable 
                               WHERE Transition_ID = @someValue) 
    AND piv.PID = @someValue