How to find people who likes each other?

I’m using mysql. There is likes_table for users. However, I am stuck to find people who like each other.

likes_table:

id (unique), from_user_id, to_user_id
1,3,1
2,1,3
3,1,64
4,1,2
5,2,1

For example, I want to find mutual likes for from_user_id =1 Therefore expected result should be:

2,3

I tried to connect two columns with IN command.

SQL code I tried:

Select to_user_id from likes_table where from_user_id IN (Select from_user_id from likes_table where to_user_id = #id)

This gives only user_id=1

Answer

One method is to filter the table for two conditions:

• Row where the from_user_id = 1; and,
• The reciprocal row exists.

The second condition can be handled using exists:

select l.to_user_id
from likes l
where l.from_user_id = 1 and
      exists (select 1
              from likes l2
              where l2.from_user_id = l.to_user_id and 
                    l2.to_user_id = l.from_user_id
             )