mysql单表查询

标签：mysql

 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

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mysql单表查询

标签：mysql

原文地址：http://wangconggao.blog.51cto.com/3253868/1579692