先用自定義變數分組排序，定義@num為序號，初始化為0，定義@last為上一行的部門，定義為null，
然後進行查詢，@num:=if(@last!=company,@num:=1,@num:=@num+1) 如果上一家公司不等於現在這家公司，就重置num為1，否則增加1
注意 需要先按公司排序 ，不然公司是亂的 序號就會不准
此時 這一步的時候，就得到了按分組排好序的資料
select id,company,salary,@num:=if(@last!=company,@num:=1,@num:=@num+1) as num,@last:=company
from
(select id,company,salary from employee ) a ,
(select @num:=0,@last:=null) b order by company,salary
) a
奇數的中位數為 （最大數目+1）/2
select count(1) as countnum,company,
case when count(1)%2=0 then count(1)/2 else null end as o1 ,
case when count(1)%2=0 then count(1)/2+1 else null end as o2,
case when count(1)%2=1 then (count(1)+1)/2 else null end as o3
from employee group by company
最後 用公司將這兩個進行關聯， 然後判斷每組的數目是奇數還是偶數來取值
select id,a.company,salary from
(select id,company,salary,@num:=if(@last!=company,@num:=1,@num:=@num+1) as num,@last:=company
from
(select id,company,salary from employee ) a ,
(select @num:=0,@last:=null) b order by company,salary
) ajoin
( select count(1) as countnum,company,
case when count(1)%2=0 then count(1)/2 else null end as o1 ,
case when count(1)%2=0 then count(1)/2+1 else null end as o2,
case when count(1)%2=1 then (count(1)+1)/2 else null end as o3
from employee group by company
) b on a.company=b.company where num=if(countnum%2=1,o3,o2) or num=if(countnum%2=0,o1,o3)
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