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