CREATE TABLE mill (id VARCHAR, architect_id VARCHAR, built_year INT)
CREATE TABLE architect (id VARCHAR, architect_id VARCHAR, built_year INT)

How many architects haven't built a mill before year 1850?
SELECT COUNT(*) FROM architect WHERE NOT id IN (SELECT architect_id FROM mill WHERE built_year < 1850)
SELECT COUNT(*) FROM "architect" WHERE NOT "id" IN (SELECT "architect_id" FROM "mill" WHERE "built_year" < 1850)