mysql中2列的唯一索引

Question

I have one table in mysql named 'UserFriends' where I am updating my websites user's friends details.

here is the schema of the table (UserFriends)

id  int,
Userid int,
friendid int,
createdate timespan

now, I want to create unique index on userid & friendid. that i have created unique index well. so, right now i am not able to insert same userid and friendid as duplicate. but if i am inserting same value in opposite field it accept without generating error.

example :

insert into userfriends ( userid, friendid )
select 1, 2  --- insert perfect
insert into userfriends ( userid, friendid )
select 1, 2  --- show error because unique index comes in a picture

now i am inserting

insert into userfriends ( userid, friendid )
select 2, 1  --- records insert here (i don't want this)

How do i prevent this?

Answer 1

First things first: for your inserts, don't use 'select' to generate the values, as it's liable to become confusing. Instead do it this way:

INSERT INTO userfriends (userid, friendid) VALUES (1, 2)

That said, the only way to have a unique qualifier that works backwards too is to do it programmatically. If you want to prevent x,y from being accepted, run SELECT * FROM userfriends WHERE userid = y AND friendid = x and if you get a result, reject the insert before it happens. If you don't, then insert into the table as normal, and your unique key will deny it from there on.