MySQL优化排名语句

先捋下变量的使用基本规则

在给一个变量赋值的同时使用这个值：

set @rownum:=0;
select actor_id,@rownum:=@rownum+1 as rownum from actor limit 10;

使用自定义变量对结果进行排名：

set @film_num:=0;
select *,@film_num:=@film_num+1 as num from(
select actor_id,count(*) as cnt from film_actor GROUP BY actor_id ORDER BY cnt DESC limit 10)t