Question

I have a table “friends” with “user1” and “user2” columns.

How can I remove duplicate relationships?
For example, if I already have
user1=1 and user2=3,
how can I remove user1=3 and user2=1 values?

Could I enforce that in MySQL with a unique index?

Answer 1

With this query you can find duplicate relationships

SELECT * 
FROM friends d
WHERE EXISTS (
SELECT 1 
FROM friends f
WHERE f.user1 = d.user2
AND f.user2 = d.user1
AND f.user2 < f.user1
)

I recommend put the lower userId for user1 and create a unique index