Mysql操作——DQL-4-字段控制查询

1. 去除重复记录

select distinct name from students;

mysql> select distinct name from students;

2.对查询字段的结果进行运算，必须都要是数据型

select *,字段1+字段2 from 表名;

列有很多记录的值为Null,因为任何东西与Null相加结果还是Null,所以计算结果可能会出现Null。

这时可以使用把Null转换成数值0的函数;

mysql> select *,age+score from students;

mysql> select *,ifnull(age,0)+ifnull(score,0) from students;

3.对查询结果起别名AS

mysql> select *,ifnull(age,0)+ifnull(score,0) as total from students;
mysql> select name as resname from students;