数据
图片说明
1. 不并列排名

SELECT order_num, quantity, @i:=@i+1 as RK
from orderitems, (SELECT @i:=0) as a
ORDER BY quantity desc;

结果
图片说明
2. 并列排名

SELECT order_num, quantity, @i:=@i+(@pre<>(@pre:=quantity)) as RK
from orderitems, (SELECT @i:=0, @pre:=-1) as a
ORDER BY quantity desc;

结果
图片说明
Tips：