SQL Optimization

Two tables: city id,name,country_id 1,xx,21 2,tt,22 3,tts,21 4,ss,31

country id,name 21,AA 22,BB 31,CC 41,EE 51,TT

The question is selecting the country name which has not attached with any city.

select * from country where id is not in (select id from country right join city where city.country_id = country.id);

But we can optimize this SQL like this:

select * from country left join city where city.country_id is null;