UNION 和在where里面写两个条件然后or起来的区别在于union会把重复的去掉，也就是用or的话如果是两个都有就会出现几次（不用distinct的话）
INTERSECT和EXCEPT不被支持，前者用join table 或者in 后者用not in
存在，所有
SELECT D.name FROM Departments D WHERE D.budget > SOME( SELECT D2.budget FROM Departments D2 WHERE D2.department_id IN (SELECT W.department_id FROM Works_in W WHERE W.employee_id = 4) );
SELECT D.name FROM Departments D WHERE D.budget >= ALL (SELECT D2.budget FROM Departments D2);
某个用exist做的判断
SELECT E.name FROM E.Employees E WHERE EXISTS (SELECT * FROM Works_in W WHERE W.department_id = 1 AND E.employee_id = W.employee_id);
这种情况下就不要吧把两个table并起来然后再where =了
关于NULL VALUE
SELECT name FROM Employees WHERE salary IS NULL；
5 + null returns null.对于四则运算
Both 5 < null , null = null return UNKNOWN.对于比较
对于 aggregate operations except COUNT(*) ignore tuples with null values on the aggregated attributes.
