Show all headquarters.
select headquarters from company
Show headquarters with a company in the banking industry.
select headquarters from company where main_industry = 'Banking'
How about headquarters with a company in oil and gas?
select headquarters from company where main_industry = 'Oil and gas'
Show headquarters with both.
select headquarters from company where main_industry = 'Banking' intersect select headquarters from company where main_industry = 'Oil and gas'