CREATE TABLE stadium (id DECIMAL, name TEXT, capacity DECIMAL, city TEXT, country TEXT, opening_year DECIMAL)
CREATE TABLE swimmer (id DECIMAL, name TEXT, nationality TEXT, meter_100 DECIMAL, meter_200 TEXT, meter_300 TEXT, meter_400 TEXT, meter_500 TEXT, meter_600 TEXT, meter_700 TEXT, time TEXT)
CREATE TABLE event (id DECIMAL, name TEXT, stadium_id DECIMAL, year TEXT)
CREATE TABLE record (id DECIMAL, result TEXT, swimmer_id DECIMAL, event_id DECIMAL)

Which countries do not have a stadium that was opened after 2006?
SELECT country FROM stadium EXCEPT SELECT country FROM stadium WHERE opening_year > 2006
SELECT "country" FROM "stadium" EXCEPT SELECT "country" FROM "stadium" WHERE "opening_year" > 2006