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1create table friendships (
2    userA int not null,
3    userB int not null,
4    primary key (userA, userB)
5);
6
7drop trigger if exists trg_friendships_order;
8delimiter //
9
10create trigger trg_friendships_order before insert on friendships
11for each row
12begin
13    declare temp int; -- should be same data type as userA & userB
14    if new.userB < new.userA then -- swap
15        set temp := new.userB;
16        set new.userB := new.userA;
17        set new.userA := temp;
18    end if;
19end
20//
21delimiter /
22	
23insert into friendships values (2, 1);
24select * from friendships;
25	
26userA | userB
27-------+-------
28     1 |     2
29	
30insert into friendships values (2, 1);
31insert into friendships values (1, 2);