Question

I have a simple SQL Server 2008 database with two tables like this:

TableA: 
    (PK)"ID"
    "Field"

and

TableB:
    (PK)"ID"
    (FK)"ID_TableA"
    "Field"

I want to select all the fields in TableA and also how many corresponding rows in TableB there are for every TableA row:

SELECT A.*, 
       COUNT(B."ID") as "B's number"
FROM "TableA" A
LEFT JOIN "TableB" B ON (A."ID" = B."ID_TableA")
GROUP BY A."ID", A."Field"

This works well, but I have this issue: if TableA is further modified (let’s say we have to add another Field2 column) I have to update the SELECT statement above to include that field in the GROUP BY clause. Otherwise I receive this error when executing the operation:

“Column ‘TableA.Field2’ is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause“

Is there a way to avoid this so I can modify my TableA without also updating all the statement like the one above?

Answer 1

You could use this (first Group By in table B, then Join to table A):

SELECT A.*, 
       COALESCE("B's number", 0) AS "B's number"
FROM "TableA" A
  LEFT JOIN 
    ( SELECT B."ID_TableA", COUNT(B."ID") as "B's number"
      FROM "TableB" B 
      GROUP BY B."ID_TableA"
    ) AS B ON (A."ID" = B."ID_TableA")