mysql中null值的排序问题分析

如下表t_user：
[table]
|name|age
|zhangsan|1
|lisi|NULL
|wangwu|2
[/table]

执行一下sql：

select * from t_user order by age;

[table]
|name|age
|lisi|NULL
|zhangsan|1
|wangwu|2
[/table]

实际上我们是想将没有填写age的记录放在最后，我们可以

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

[table]
|name|age
|zhangsan|1
|wangwu|2
|lisi|NULL
[/table]

为什么会这样？可以这样来理解：

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

等价于:

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

看出来了吗？