Question

I’ve been given a query like this

SELECT * FROM dayTable
LEFT JOIN ProcessTable firstprocess
ON dayTable.id = a.daytablefkid 
AND firstprocess.processid in (1234)
AND firstprocess.daytablefkid 
     IN (SELECT id 
         FROM daytable  
         WHERE dayTable.DateFrom > @dateFrom 
         AND dayTable.DateTo < @dateTo)

LEFT JOIN ProcessTable secondprocess
ON daytable.id = secondprocess.daytablefkid
and secondprocess.processid in (4567)
    IN (SELECT id 
        FROM daytable 
        WHERE dayTable.DateFrom > @dateFrom 
        AND dayTable.DateTo < @dateTo)


WHERE dayTable.DateFrom > @dateFrom AND dayTable.DateTo < @dateTo

tHe first thing i noticed is that the first IN-statement “IN(XXXX)” in the joins can be converted to a WHERE-statement, but the second IN statement in both joins is a bit confusing because the same filtering is also done at the end on the query.

What I’m wondering if they are indeed redundant or can the resultset be different if they are removed?

Answer 1

Yes, the IN (SELECT id FROM daytable WHERE dayTable.DateFrom > @dateFrom AND dayTable.DateTo < @dateTo)-parts are redundant, since you already joined on the daytable, on which you have the same WHERE-clause.

This query should give the same results:

SELECT * 
FROM dayTable
LEFT JOIN ProcessTable firstprocess
  ON dayTable.id = firstprocess.daytablefkid 
    AND firstprocess.processid = 1234
LEFT JOIN ProcessTable secondprocess
  ON daytable.id = secondprocess.daytablefkid
    AND secondprocess.processid = 4567
WHERE 
  dayTable.DateFrom > @dateFrom 
  AND dayTable.DateTo < @dateTo