查询数据(DQL)-select

查询数据(DQL)-select：

        1、选择特定的字段
              select id,name from user;
        2、给字段取别名-as
              select id,name from user;
              select id maoxian,name from user;
              select id as maoxian,name from user;
        3、distinct关键字的使用
              select distinct age from user;
        4、使用where条件进行查询
              select * from user where id>=3 and id<=5;
        5、查询空值
              select * from user where age is null;
              select * from user whrer age is not null;
        6、between and的使用
              select * from user where id between 3 and 5;
        7、in的使用方法
              select * form user where id=1 or id=2 or id=10;
              select * from user where id in(1,2,10);
        8、like的使用方法
              _匹配一个字符
              //%在前，name这一列的索引会失效
              select * from user where name like "%mysql%";
        9、regexp正则检测
              select * from user where username regexp 'php';
              select * from user where username regexp '^php';
              select * from user where username regexp 'php$';
      10、使用order by对查询结果排序
              select * from user order by id asc;//默认就是升序，数字从小到大 
              select * from user order by id desc;//升序变为降序，数字从大到小