Question: I have a User table that contains User ID and Name.

I have a Likes table that contains UserID1 and UserID2.

When User1 likes User2, their IDs are entered in to the likes table respectively.

How can I retrieve all users that BOTH like each other?

Answer: 
Join the likes table with itself and select the rows with mutual likes:

```
    select * 
      from Likes like1 
inner join Likes like2 on like1.UserID1 = like2.UserID2 
                      and like1.UserID2 = like2.UserID1

```
Use self join

```
select t1.* from Likes as t1 
inner join Likes as t2  on t1.UserID1 = t2.UserID2 and t1.UserID2 = t2.UserID1

```