MySQL 数据库 索引优化

Question

现有sql语句：
①select * from tab1 where number = '5' and mode >= 1 and mode <= 21 order by id desc limit 100;

②select * from (select * from tab2 where mode >= 1 and mode <= 21 and name = '5' and create_time >= '2018-01-01' and create_time <= '2019-09-04' order by id desc) A order by id;

请问这两句sql该如何建立索引以达到优化的目的？对于①，我建立了联合索引，key（number，id）但是explain的时候提示没有用到该索引，用的是主键索引。

Answer 1

SQL1：key (number,mode)
SQL2为什么外层要排序，直接在子查询里order by id不行么？