SELECT count(*) FROM architect WHERE id NOT IN ( SELECT architect_id FROM mill WHERE built_year  <  1850 );
How many architects haven't built a mill before year 1850?
SELECT count ( * ) FROM architect WHERE id NOT IN ( SELECT architect_id FROM mill WHERE built_year < 1850 ) ;
select count ( * ) from architect where id not in ( select architect_id from mill where built_year < value )
How many architects have n't built a mill before year 1850 ?