count(*)加group by是查询的分组后每个城市的数量,不存在时结果会返回空值,这时可以使用NVL来消除空值。
SELECT la.city ,COUNT(*) FROM employees ea,departments da,locations la WHERE ea.department_id=da.department_id AND da.location_id=la.location_id AND la.location_id=2400 AND ea.salary>6500 group by la.city;