Question

The query is this one:

SELECT FriendID FROM Relationships WHERE UserID = 1
INTERSECT
   (SELECT FriendID FROM Relationships WHERE UserID = 2
    UNION SELECT UserID FROM Relationships WHERE FriendID = 2)

(for the curious readers, please note that the friend relationship is not necessarily symmetrical in this scenario)

I’ve tried all the possible combination of parentheses with no luck.

If I omit the parentheses, there’s no operator precedence in the sense that it reads it like 5+6*3 = 33, so if I put the union before the intersection, the query works fine. But what will I do when I will have to intersect two unions?

Answer 1

Thanks to Larry Lustig (which pointed me this), I rewrote my query as follows

SELECT FriendID FROM Relationships WHERE UserID = 1
INTERSECT SELECT ID FROM
   (SELECT FriendID AS ID FROM Relationships WHERE UserID = 2
    UNION SELECT UserID AS ID FROM Relationships WHERE FriendID = 2)

And it works.