特殊比较运算符
select * from employees where department_id is null;
select * from employees where department_id is not null;
select * from employees between 5000 and 10000;
select * from employees not between 5000 and 10000;
select * from employees where department_id in (10,20,50);
select * from employees where department_id not in (10,20,50);
select * from employees where department_id not in (10,20,50,null);--逻辑上，只有一个为假，则全部为假，所以没有出现结果