MySQL: Using union to check if two columns contain eachother's values?

To avoid "duplicate" rows.. e.g. 2 rows for every couple of friends, add the condition a. Uid.

SELECT a. Uid, a. Fid FROM friends a JOIN friends be ON a.

Fid = b. Uid AND a. Uid = b.

Fid WHERE a. Uid = 1 AND a. Fid = 2 To avoid "duplicate" rows.. e.g.2 rows for every couple of friends, add the condition a.

Uid Uid, a. Fid FROM friends a JOIN friends be ON a. Fid = b.

Uid AND a. Uid = b. Fid AND a.

Uid Fid = 2 So, to count all friendships, use: SELECT COUNT(*) AS countAllFrienships FROM friends a JOIN friends be ON a. Fid = b. Uid AND a.

Uid = b. Fid AND a. Uid Uid So, to count friends of user X, use: SELECT a.

Uid , COUNT(*) AS numberOfFriends FROM friends a JOIN friends be ON a. Fid = b. Uid AND a.

Uid = b. Fid WHERE a. Uid = X GROUP BY a.uid.

Thank you for these examples, I enjoy picking at them to learn. – Den R. Jun 30 '11 at 10:45.

You need an auto join to return all the couple user/friend which are in a bijection friendship : SELECT f. Uid, f. Fid FROM Friend f INNER JOIN Friend ff ON f.

Uid = ff. Fid AND f. Fid = ff.

Uid Be aware that it will return 'duplicates', in you case : uid | fid 5 6 6 5.

Select count(*) from friends where ((uid = 1) and (fid = 2)) or ((uid = 2) and (fid = 1)) Note that you have to do a count(*), as the UID's will be different if there's a mutual friendship. You'd get two rows with a count of 1, instaed of a single row with a count of 2.

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