Question: Among the trains running east, how many trains have at least 4 cars?
SQL: SELECT SUM(CASE WHEN T1.direction = 'east' THEN 1 ELSE 0 END)as count FROM trains AS T1 INNER JOIN ( SELECT train_id, COUNT(id) AS carsNum FROM cars GROUP BY train_id ) AS T2 ON T1.id = T2.train_id WHERE T2.carsNum >= 4