SQL count many-to-many relations in common

Question

i have a sql problem with this schemas :

table user(id, name) table page(id, name) table page_user(id_page, id_user)

one page can be liked by multiple users and one user can like multiple pages

i would like to select all users in my table with a column with the number of liked page in common with some fixe user like : id_user | page_in_common_with_user_#id#_count

Answer 1

Assuming that "liked page" refers to the page_user table, then you can use a self join:

select pu.id_user, count(pux.id_page) as PagesInCommonWithX
from page_user pu left join
     page_user pux
     on pu.id_page = pux.id_page and
        pux.id_user = $x
group by pu.id_user;