VanZan

В MySQL, начиная с версии 8.0, вы можете использовать оконную функцию row_number():

select 
	class,
	price
from (
  select 
	  class,
	  price,
	  row_number() over (partition by class order by price) rn
  from complex
) prices
where 
	(class=1 and rn < 3) -- 2 econom
	or (class=2 and rn < 4) -- 3 comfort
	or (class=3 and rn < 2) -- 1 premium
;

Mysql online