Question

Can someone please help me on a sql query where there’s a table such as

Col A | Col B
-------------
A       B
C       D
E       F
B       A

how do I remove {B, A} or select {B, A}

The question is:

If two students A and B are friends, and A likes B but not vice-versa,
remove the Likes tuple.

I have figured out the table where A and B are friends and A likes B, but I don’t know how to remove the B likes A tuple

I have

select friend.id1, friend.id2
from friend, likes
where friend.id1 = likes.id1 and friend.id2 = likes.id2

Answer 1

I’ve seen this homework assignment before…

DELETE FROM Likes
WHERE ID1 IN (SELECT L.ID1
                       FROM Friend F,Likes L
                       WHERE F.ID1 = L.ID1 AND F.ID2 = L.ID2
                       AND F.ID1 NOT IN (SELECT ID2 
                                         FROM Likes
                                         WHERE ID1=L.ID2 AND ID2=L.ID1))

Try to figure these out on your own — or if you can’t, try searching Google.