Table schema:
CREATE TABLE museum (Museum_ID VARCHAR, open_year INTEGER); CREATE TABLE visitor (id VARCHAR); CREATE TABLE visit (visitor_id VARCHAR, Museum_ID VARCHAR)

Question: Find the number of visitors who did not visit any museum opened after 2010.
SQL: SELECT COUNT(*) FROM visitor WHERE NOT id IN (SELECT t2.visitor_id FROM museum AS t1 JOIN visit AS t2 ON t1.Museum_ID = t2.Museum_ID WHERE t1.open_year > 2010)