How many membership registrations are there in file?
select count(*) from membership_register_branch
Show me the id of branches in any of them.
select branch_id from membership_register_branch
Give me the name of branches which are not one of those.
select name from branch where branch_id not in (select branch_id from membership_register_branch)
What about their cities?
select name, city from branch where branch_id not in (select branch_id from membership_register_branch)