How many different markets are there?
select count ( distinct market_id ) from phone_market
How many different phones are there?
select count ( distinct name ) from phone
Which phone is sold in the most different markets?
select t1.name from phone as t1 join phone_market as t2 on t1.phone_id = t2.phone_id group by t1.phone_id order by count ( * ) desc limit 1
How many phones are not available in any markets?
select count ( name ) from phone where phone_id not in ( select phone_id from phone_market )
What is the name of that phone?
select name from phone where phone_id not in ( select phone_id from phone_market )