Question

I’m struggling to find a query which will return members who aren’t friends of a certain member. Here is the layout of my tables:

member_login:

MemberID, email, password

member_info:

memberID, first_name, last_name

member_friends:

friendID, req_member, req_date, app_member, app_date, date_deactivated

I tried to use NOT IN to run a query that would return the opposite of friends but nothing I try seems to be working. Here’s what I thought would work:

SELECT Mi.First_Name, Mi.Last_Name
  FROM Member_Info Mi
 WHERE Mi.Memberid NOT IN(
                           SELECT Mi.Memberid, Mi.First_Name, Mi.Last_Name
                             FROM Member_Info Mi, Member_Login Ml, Member_Friends Mf
                            WHERE Mi.Memberid = Ml.Memberid
                               AND (Mi.Memberid = Mf.Req_Member
                                   AND Mf.App_Member = 1
                                   OR Mi.Memberid = Mf.App_Member
                                   AND Mf.Req_Member =1)
                               AND Ml.Date_Deactivated <= 0
                               AND Mf.App_Date > 0
                          );

Any ideas?

Answer 1

what if you try

SELECT Mi.First_Name, Mi.Last_Name
  FROM Member_Info Mi
 WHERE Mi.Memberid NOT IN(
                           SELECT Mi.Memberid
                              FROM Member_Info Mi, Member_Login Ml, Member_Friends 
                            WHERE Mi.Memberid = Ml.Memberid
                               AND (Mi.Memberid = Mf.Req_Member
                                   AND Mf.App_Member = 1
                                   OR Mi.Memberid = Mf.App_Member
                                   AND Mf.Req_Member =1)
                               AND Ml.Date_Deactivated <= 0
                               AND Mf.App_Date > 0
                          );

because you are returning three values in your NOT IN ( … ) when it should be one

SELECT Mi.Memberid, Mi.First_Name, Mi.Last_Name

should be

SELECT Mi.Memberid

you might get another error about your FROM statement as you will need to join to tables together to compare values but your friend table does not have any matching values so this could be a problem (poor design)

EDIT: database designs

A lot of this comes down to experience really but there are a few good books which you can check out like Head First SQL and Relational Database Design Clearly Explained (The Morgan Kaufmann Series in Data Management Systems) (Paperback), these are great books to learn the basics and basic design, i recommend you check them out