sql 查询性能问题 case when

1、 select *,a.a2,case when a.a2='1' then '是' when a.a2='2' then '也许是' when a.a2='3' then '不是' ) end maybe from a left join b on a.a1 = b.b1 where a.a2 = '1'

2、 select * from (select *,a.a2,case when a.a2='1' then '是' when a.a2='2' then '也许是' when a.a2='3' then '不是' ) end maybe from a left join b on a.a1 = b.b1） t where maybe = '是'

以上两个查询sql，查询的结果是一样的，请问在执行顺序上有什么不同？
第2比第1句快了几十倍。不知为什么

'不是'后面多了个“)”吧。2中不加条件join怎么还会比1中加条件where a.a2='1'的join快呢？

--创建表 create table total (  brand varchar(50) not null,  quantity int null,  price int null ) --查询语句 select brand as 品牌, case when price < 500 then '500以下'   when price > 500 and price < 1000 then '500-1000'   when price > 1000 then '1000以上'   else null end as 级别, case when price < 500 then sum(quantity)   when price > 500 then sum(quantity)   when price > 1000 then sum(quantity)   else 0 end as 数量, avg(price) as 平均值 from total group by brand,price

最终结果如图所示