mysql中null值的排序问题分析
如下表t_user：nameagezhangsan1lisiNULLwangwu2
执行一下sql：
select * from t_user order by age;
nameagelisiNULLzhangsan1wangwu2
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;
nameagezhangsan1wangwu2lisiNULL
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？
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