Mysql 排序null值 排序问题分析

mysql中null值的排序问题分析

 

如下表t_user： 

name age

zhangsan 1

lisi NULL

wangwu 2

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执行一下sql： 

Sql代码  

select * from t_user order by age;  

 

name age

lisi NULL

zhangsan 1

wangwu 2

 

实际上我们是想将没有填写age的记录放在最后，我们可以 

Sql代码  

select * from t_user order by age is null, age;  

 

name age

zhangsan 1

wangwu 2

lisi NULL

 

为什么会这样？可以这样来理解： 

Sql代码  

select * from t_user order by age is null, age;  

 

等价于: 

Sql代码  

select * from (select name, age, (age is null) as isnull from t_user) as foo order by isnull, age;