1、查询出某条件下的所有信息
select * from student where age = 18
2、求和
select sum(score) from student where name="lilei"
3、求平均数
select avg(score) from student where name="lelei"
4、对查询结果排序（倒叙：desc ）
select * from student where name = "lilei" order by score desc
5、求总数
select count(*) from student where class=“one”
6、求总数（去重）
select count(distinct id) from student where class="one"
7、根据一个或多个列表字段进行分组统计查询（where>分组>聚合函数）
select class,count(distinct age) from student where age>19 group by class
8、两个表连接，并去掉重复的列
select a.*,b.address from student a join info b on a.id=b.id
9、嵌套查询
select a.id,a.name,(select b.address from info b where a.id=b.id) address from student a
select a.id,a.name,b.address from student a,info b where a.id=b.id
二、插入、修改和删除
1、插入多条数据
insert into student values
(1,"stuname1",18,"Maths","class1"),
(2,"stuname2",88,"Maths","class2")
2、修改
update student set score=92 where name="kim" and subject="Maths"
3、删除
delete from student where name="kim" and subject="English"