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>I have a table called friends with a user_id and a friend_id (both of these relate to an id in a users table). >For each friend relationship there are two rows. There are currently ONLY reciprocal relationships. So if user ids 1 and 2 are friends there will be two rows (1,2) and (2,1). >For 2 arbitrary ids, I need a query to get two pieced of data: > * Are the two users friends? > * How many friends do the two users have in common. >Is there a way to do this with one query? Currently I've only been able to figure out how to do it with two. >SELECT > EXISTS( > SELECT 1 > FROM friends > WHERE user_id = 166324 AND friend_id = 166325) AS friends, > (SELECT COUNT(1) > FROM friends f1 JOIN friends f2 ON f1.friend_id = f2.friend_id > WHERE f1.user_id = 166324 AND f2.user_id = 166325) AS mutual; >I'm wondering if there is a better way to do this using only one query. I've tried a couple of GROUP BY approaches but they haven't worked. > Hi, this should do the job, but requires an additional check constraint user_id <> friend_id to be on the safe side: SELECT count(case when c1=1 then true end)=1 as are_friend, count(*)-1 as common_friends FROM ( SELECT count(*) as c1 FROM friends WHERE user_id IN (USER1, USER2) GROUP BY case when user_id = USER2 then USER1 else USER1 end, friend_id HAVING COUNT (*) =2 OR COUNT(case when friend_id =USER1 then true end)=1 ) q1 regards, Marc Mamin |
