SQL to find common traits among all users

Question

I'm trying to retrieve just the common traits from all the users.

  User(u)Trait(t)
  u1      t1
  u1      t2
  u1      t3
  u2      t2
  u2      t3
  u2      t4
  u3      t2
  u3      t3
  u3      t4

I'm trying to join the table to itself but not getting the desired output.

I expect the output of the above to be t2 and t3 and these are the only traits that are present in all the 3 users.

Answer 1

You can use aggregation:

select ut.trait
from usertraits ut
group by ut.trait
having count(*) = (select count(distinct ut2.user) from usertraits ut2);