select * from stu where age>20;-- 1
select * from stu where age>=20;-- 2
select * from stu where age>=20 and age<=30;-- 3
select * from stu where hire_date BETWEEN '1998-09-01' and '1999-09-01';-- 4
select * from stu where age=18;-- 5
select * from stu where age<=18;-- 6
select * from stu where age in(18,20,22);-- 7
select * from stu where english is null;-- 8
select * from stu where name like '馬%';-- 1
select * from stu where name like '_花%';-- 2
select * from stu where name like '%德%';-- 3
