mysql中虚拟列是什么意思（直叙说）？

虚拟列就是人为增加的列：set @rn=0select (@rn:=@rn+1) rn,name from (select id,name from tasks order by id)name

第一种方法：

select (@i:=@i+1) as i,table_name.* from table_name,(select @i:=0) as it

第二种方法：

set @rownum=0

select @rownum:=@rownum+1 as rownum, t.username from auth_user t limit 1,5

现在要按年龄从大到小取出前5条数据的话，可用如下语句：

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select 姓名,年龄

from

(select 姓名,年龄,row_number() over (order by 年龄 desc) as rn from student) t

where rn<=5

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