How many different industries have companies have headquarters in USA?
select count(distinct industry) from companies where headquarters = "USA"
headquarters in China?
select count(distinct industry) from companies where headquarters = "China"
how about both?
select count(*) from (select industry from companies where headquarters = "USA" intersect select industry from companies where headquarters = "China")
Which ones?
select industry from companies where headquarters = "USA" intersect select industry from companies where headquarters = "China"