Question

I have a table called friends which has id and name and a self join table called friendship which stores the relationship which includes friend_id and friend2_id .
how do i get the names of related friends if a name of a particular frnd is given

example

id name
1  jack
2  kurt
3  jim

and

friendship
f_id f1_id
1    3 

So if i give ‘jack‘ i should get jim back

Answer 1

You could do this in one query or two queries, depending on what you want to accomplish.

A simple one could be:

SELECT
  f_id,
  f1_id
FROM
  friendship
WHERE
  f_id=1
  OR
  f1_id=1

And then you can get the specific friends with a statement like:

SELECT name FROM people WHERE id IN(2,3)

Alternative is a self join but the hard part here is that your id might be in both f_id and f1_id so that would need some UNION command or something like (untested):

SELECT
  p1.name,
  p2.name,
FROM
  friendship
  INNER JOIN
    people AS p1
      ON friendship.f_id = people.id
  INNER JOIN
    people AS p2
      ON friendship.f1_id = people.id
WHERE
  p1.id=1 OR p2.id=1

I would thoroughly check the speed of these options since they are quite heavy on huge amounts of records. If you measure you need more performance try some alternative. For example when you always put the smallest people.id in f_id and the bigger one in f1_id you might run 2 queries which you union. Alternative is to denormalize a small bit to cache the results if you need them frequently.

It would save you lots of joings for example if you would add the names into the friendship table:

SELECT
  f_id,
  f1_id,
  f_name,
  f1_name
FROM
  friendship
WHERE
  f_id=1
  OR
  f1_id=1