简单查询：

select * from tb_name

select field1,field2 from tb_name;

select * from tb_name where qualification ;

例子：select  name,age from students where age+1>20;

select name from student where age>20 and gender='M';

select name from student where age>20 or gender='M';

select name from student where not age>20 and not gender='M';

select name from student where not (age>20 or gender='M');

select name from student where age>=20 and age<=50;或者select name from student where age between 20 and 25;

select name from student where like name 'Y%';

REGEXP 或者RLIKE 后面可以跟上正则表达式：

select name from student where name relike '^[MNY].*$';匹配以M或者N或者Y开头后面任意字符的。

IN的用法：年龄在18、20、25岁的同学名字

select name from student where age in (18,20,25)；

显示student表中cid是null的同学；

select name from student where cid is null;

不是null值的写法：

select name from student where cid is not null;

根据查询到的数据排序：

select name from student where cid is not null order by name;

数据存储的三种格式：堆文件、顺序文件、hash

给字段取别名；

select name as student_name from student where cid is not null order by name;

限制显示的行数(limit)；

例子： select name as student_name from student as xiaohai  limit 2

限制只显示两行。

略过前两行：

select name as student_name from student as xiaohai limit 2,3

略过前两行显示三行。

聚合计算：

所有同学的平均年龄：

select avg(age) from student；

select max(age) from student;

select min(age) from student;

select sum(age) from student; 求和

select count(age) from student;   求个数

group by ：分组

select age,gender from student group by gender;

按照性别把student表分组(即男，女) 显示age gender

select avg(age) from student group by gender;

按照性别把表student分组，分别计算出他们的的平均值。

分组的主要目的是聚合：

注意：对group by 条件进行过滤必须用having 它的作用相当于where

select count(cid1) as persons,cid from student group by cid1 having persons>=2;

having的作用是对group by的结果做再次过滤