Show all the main industries for companies.
select main_industry from company
Order them in descending order by total market value.
select main_industry from company group by main_industry order by sum(market_value) desc
What one has the highest?
select main_industry from company group by main_industry order by sum(market_value) desc limit 1
Also show its number of companies.
select main_industry, count(*) from company group by main_industry order by sum(market_value) desc limit 1