mysql中null值的排序问题分析
如下表t_user：
select * from t_user order by age;
select * from t_user order by age is null, age;
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;