您可以使用
UNIOn来获得用户朋友：
SELECt User2 UserId FROM friends WHERe User1 = 1 UNIOn SELECt User1 UserId FROM friends WHERe User2 = 1
然后，
UNIOn为两个不同的用户加入其中的两个，
UserId即可获得共同的朋友：
SELECt UserAFriends.UserId FROM( SELECt User2 UserId FROM friends WHERe User1 = 1 UNIOn SELECt User1 UserId FROM friends WHERe User2 = 1) AS UserAFriendsJOIN ( SELECt User2 UserId FROM friends WHERe User1 = 2 UNIOn SELECt User1 UserId FROM friends WHERe User2 = 2) AS UserBFriends ON UserAFriends.UserId = UserBFriends.UserId
