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;
Mysql 排序null值 排序问题分析
原文：http://www.cnblogs.com/colmeluna/p/4689311.html
