SQL - Select 具有相同数量的用户对

Question

I'm trying to write a query that returns pairs of users that have the same number of loans.

I have this table:

LOANS

id_loan  book_barcode id_user
1         123            1
2         321            2
3         456            3
4         678            4
5         721            1
6         934            2

That's my code how to get pairs of users:

SELECT l1.id_user user_1, l2.id_user user_2
FROM loans l1
JOIN loans l2 ON l2.id_user > l2.id_user
GROUP BY l2.id_user, l2.id_user; 

This is what I want from my query, but I don't know how to compare in the right way two counts, I tried but it didn't work.

id_user1   id_user2  number_loan
 1            2           2
 3            4           1

Answer 1

The most reasonable way is to aggregate the loans before joining:

with u as (
      select id_user, count(*) as num_loans
      from loans l
      group by id_user
     )
select u1.id_user, u2.id_user
from u u1 join
     u u2
     on u1.num_loans = u2.num_loans and u1.id_user < u2.id_user;

You could do this without pre-aggregating, but the query will be much more expensive:

select u1.id_user, u2.id_user
from loans u1 join
     loans u2
     on u1.id_user < u2.id_user
group by u1.id_user, u2.id_user
having count(distinct u1.id_loan) = count(distinct u2.id_loan);

I don't recommend this approach.