Question

I’m taking a database course and I’ve got stuck on one of the questions. “For all cases where A is friends with B, and B is friends with C, add a new friendship for the pair A and C”.
This is as far as I’ve got

INSERT INTO 
    friend
SELECT DISTINCT 
    f1.ID1, f2.ID1 
FROM 
    friend f1 
    JOIN friend f2 using (ID2), 
    friend 
WHERE 
    f1.ID1 <> f2.ID1 
AND friend.ID1 <> f1.ID1 
AND friend.ID2 <> f1.ID2

The schema is here http://www.sqlfiddle.com/#!5/cf8b5/23

I wonder if somebody could give me a few hints on how to proceed.
Thanks.

Answer 1

I think you have the basic concepts.

I’d do it like the following:

INSERT INTO friend
(ID1, ID2)
SELECT DISTINCT f1.ID1, f2.ID2
FROM friend f1
  INNER JOIN f2
    ON f1.ID2 = f2.ID1
      AND f1.ID1 <> f2.ID1
      AND f1.ID2 <> f2.ID2
WHERE f1.ID1 <> f2.ID2