Re: Combining two queries
| От | John McKown |
|---|---|
| Тема | Re: Combining two queries |
| Дата | |
| Msg-id | CAAJSdjhYfZ30QhgFdvCC8ujuaET=1R6TqrYRXaNUV-nCuMQObg@mail.gmail.com обсуждение исходный текст |
| Ответ на | Combining two queries (Robert DiFalco <robert.difalco@gmail.com>) |
| Ответы |
Re: Combining two queries
(John McKown <john.archie.mckown@gmail.com>)
|
| Список | pgsql-general |
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.SELECTEXISTS(SELECT 1FROM friendsWHERE user_id = 166324 AND friend_id = 166325) AS friends,(SELECT COUNT(1)FROM friends f1 JOIN friends f2 ON f1.friend_id = f2.friend_idWHERE 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.
This appears, to me, to require a RECURSIVE CTE. Similar to the description on http://www.postgresql.org/docs/9.1/static/queries-with.html towards the bottom, when it goes into avoiding loops on parts which are made up of sub-parts which are themselves sub-parts to other parts. In your case, this would be to eliminate multiple friendship paths which lead to a given person. I.e. A friend of B, friend of C, friend of D, friend of B leading to a recursive loop. In particular, the example:
WITH RECURSIVE search_graph(id, link, data, depth, path, cycle) AS ( SELECT g.id, g.link, g.data, 1, ARRAY[g.id], false FROM graph g UNION ALL SELECT g.id, g.link, g.data, sg.depth + 1, path || g.id, g.id = ANY(path) FROM graph g, search_graph sg WHERE g.id = sg.link AND NOT cycle ) SELECT * FROM search_graph;
Could be a template for you to start with. Where "id" is the "user_id" and "link" is the "friend_id". You could use that CTE to create a VIEW where "search_graph" is "friends_of_friends". I don't have an exact query for you, sorry. You then use the VIEW to do something like:
-- number of friends in common:
SELECT COUNT(*) FROM (
SELECT friend_id FROM friends_of_friends WHERE user_id = 166324
INTERSECT
SELECT friend_id FROM friends_of_friends WHERE user_id = 166325
)
-- Are two people direct friends:
SELECT user_id, friend_id FROM friends
WHERE user_id = 16634 AND friend_id = 166325
OR user_id = 166325 AND friend_id = 166324;
If you want a "transitive" friendship, use the friends_of_friends view instead of the friends table.
--
While a transcendent vocabulary is laudable, one must be eternally careful so that the calculated objective of communication does not become ensconced in obscurity. In other words, eschew obfuscation.111,111,111 x 111,111,111 = 12,345,678,987,654,321
John McKown
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