Question

Without the third join D.cid = C.id, this query gives me the count of C. With the third join it corrupts the count and gets unwanted tuples into the count of C’s join. So how can I get the count of C and D without having the C count effected? Is there a form of parenthesis I can use to make sure I get the correct count?

SELECT A.*, B.*, COUNT(C.aid) AS cCount 
FROM tableA A

LEFT JOIN tableC AS C ON A.id = C.aid
INNER JOIN tableB AS B ON A.id = B.aid
LEFT JOIN tableD AS D ON D.cid = C.id

GROUP BY  A.id

Answer 1

I would have the counts from the other tables pre-aggregated unto themselves and joined… something like…

SELECT 
      A.*, 
      B.*, 
      COALESCE( PreAggC.CCount, 0 ) as CCount,
      COALESCE( PreAggC.WithDCount, 0 ) as WithDCount
   FROM 
      tableA A

         JOIN tableB B
            on A.ID = B.aID

         LEFT JOIN ( select aID, 
                            count( distinct id ) CCount,
                            count(*) as WithDCount
                        from tableC
                           left join tableD D
                              on c.ID = D.cID
                        group by aID ) PreAggC
            on A.id = PreAggC.aID

Now, do you really want how many entries actually have “D” records? so I included both counts… distinct “C” entries, and the overall count with correlation with “D”