SQL get unique month year combos

Question

SELECT     MONTH(sessionStart) AS Expr1, YEAR(sessionStart) AS Expr2
FROM         tblStatSessions
WHERE     (projectID = 187)
GROUP BY sessionStart

This returns:

11 | 2010

11 | 2010

11 | 2010

12 | 2010

12 | 2010

But I need it to only return each instance once, IE:

11 | 2010

12 | 2010

If that makes sense!

Answer 1

The following should be what you want:

SELECT     MONTH(sessionStart) AS Expr1, YEAR(sessionStart) AS Expr2
FROM         tblStatSessions
WHERE     (projectID = 187)
GROUP BY MONTH(sessionStart), YEAR(sessionStart)

in general you need to group by every non-aggregate column that you are selecting. Some DBMSs, such as Oracle, enforce this, i.e. not doing so results in an error rather than 'strange' query execution.