Question

Hi I need some help with the following scenario in php. I have a db with users every user has an ID, have_card and want_card. I know how to make a direct match (one user trades with another user). But if there is no direct match but there is a circular swap like:

User #1 has card A wants card B

User #2 has card B wants card C

User #3 has card C wants card A

In this scenario there is no direct match between two users. But if:

User #1 gives his card to User #3

User #3 gives his card to User #2

User #2 gives his card to User #1

Every ones happy.

All the info I have to start with is User #1 how do I find User #2 and User #3?

Thanks to everyone for your answers.

Answer 1

This is a challenging scenario. I had to build this for a book swapping application a while back and using a white board I was able to more easily visual and solve the problem.

What I did was use the same table inside the query multiple times, just using a different name.

In my scenario, the books that were needed were in a separate table from the books the users had. Hopefully you’ll be able to see the logic in this query:

$query = 'SELECT          A.Owner_ID as Owner_1, C.Owner_ID as Owner_2, E.Owner_ID as Owner_3      FROM         E_User_Books_Have as A, E_User_Books_Have as C, E_User_Books_Have as E,         E_User_Books_Needed as B, E_User_Books_Needed as D,E_User_Books_Needed as F      WHERE          A.Book_ID=D.Book_ID AND         C.Book_ID=F.Book_ID AND         E.Book_ID=B.Book_ID AND          A.Owner_ID='$ME' AND B.Owner_ID='$ME' AND A.ID='$ID'AND          C.Owner_ID=D.Owner_ID AND           E.Owner_ID=F.Owner_ID AND         C.Owner_ID!='$ME' AND         E.Owner_ID!='$ME'';