select Count( p1.name) , u1.email from users as u1 INNER JOIN
purchases as p1 ON u1.id=p1.user_id
where u1.email IN (select u.email from users as u
INNER JOIN purchases p ON u.id=p.user_id
where p.name='Williams Selden') group by u1.email;

count |           email
-------+----------------------------
    15 | Shanell.Maxson@gmail.com
    23 | Ozella.Yoshimura@gmail.com
    14 | Isabel.Breeding@gmail.com

select DISTINCT p1.name , u1.email from users as u1 INNER JOIN
purchases as p1 ON u1.id=p1.user_id  where u1.email
IN (select u.email from users as u INNER JOIN purchases p ON
u.id=p.user_id where p.name='Williams Selden') group by u1.email,
p1.name;