MySQL Insert if select is null to avoid bidirectional duplicates - mysql

Question

I'm building a db to hold friendships between users of my app.
The server I use to communicate with the MySQL instance is written using Node.js (Express).
My table 'friendships' consists mainly of two INTs which correspond (foreign keys) to user ids.
I want to avoid bidirectional duplicates ( 1,2 vs. 2,1) so I need to write a query which does the following:
INSERT INTO friendships f (id_1, id_2) VALUES (?, ?) IF (SELECT * FROM friendships s WHERE s.id_1=? AND s.id_2=?) IS NULL ;
Obviously this one doesn't really work. And of course I would have the last two question marks have opposite values compared to the first ones, and a UNIQUE key on the ids (id_1, id_2).
The usual answer for these kind of questions is "just order your ids by size to avoid duplicates" and it's a good answer. But in my case, I want to keep record of who sent the friend request (and who approved), without using any extra variables (and extra queries).
Also, I don't want to use code for this, in order to avoid "concurrent" problems.
Thanks!

Answer 1

In MySQL, you can do this with a trigger that does the check. Some other databases have functional indexes, indexes on computed columns, or check constraints that help implement this functionality.
If you want to do the check in the insert, you can do:
INSERT INTO friendships(id_1, id_2)
select new1, new2
from (select ? as new1, ? as new2) t
where not exists (select 1
from friendships f
where f.id_1 = new2 and f.id_2 = new1
);
You should also have a unique index on id_1 and id_2:
create unique index idx_friendsships_id1_id2 on (id_1, id_2);
EDIT:
The basic query is:
INSERT INTO friendships(id_1, id_2)
select ?, ?
from dual
where not exists (select 1
from friendships f
where f.id_1 = ? and f.id_2 = ?
);
But you have to get the arguments in the right order, so the earlier method is less prone to error.