How many stadiums are there in file?
select count(*) from stadium
What about in terms of different countries?
select country, count(*) from stadium group by country
Show me the countries that have stadiums with capacity greater than 60000.
select country from stadium where capacity > 60000
Among them, which ones also have stadiums with capacity less than 50000.
select country from stadium where capacity > 60000 intersect select country from stadium where capacity < 50000