select name,country from tb;
select * from tb;
select distinct country from tb;//筛选出不重复的唯一值
select * from tb where id="1"
select * from tb where num=1;
select * from tb where num > 3 and num < 5;
select * from tb where num > 3 or id < 6;
select * from tb where not num > 3; //小于等于3
select * from tb where id is null ;//id 是空值
select * from tb where num between 3 and 6 ;
select * from tb where num in (2,5,6); //num 等于2,5,6的
select * from tb where country like "N%";//以N开头的国家； %N%含有N的国家； %N_倒数第二个字是N的国家 （%多个字值；_下划线一个字符）
select * from tb where num > 5 and (country = "USA" or country = "UK")
select * from tb order by num ASC|DESC //升序、降序
select * from tb order by country,id //ORDER BY 多列的时候，先按照第一个country排序，在按照第二个id排序
insert into tb (name,num,country) values ('ss','5','US') //没有指定的列默认是0
update tb set num= '6',country='china' where name='goole'
delete * from tb
delete from tb where name="baidu” and country="US"