Find intersection between previous values

[agni_gold]

Hi Friends,

I have one situation ,

I have below data for Month and ID

Month	ID	Required_Output
Dec-15	3	null
Jan-16	3	null
Feb-16	3	3
Mar-16	4	null
Apr-16	4	null
May-16	4	4

Now i want each row should  compare its previous 3 rows and find intersection value , i have placed required output in one column.

Example : For Feb-2016  , it will compare Dec-2015,Jan-2016 and Feb-2016 any ID: 3 is common in between , so 3 should return in front of Feb-2016 , else null

[vinieme12]

Attached updated App

SUM(AGGR(IF(ID = rangesum(Above(ID,0,3))/3 , ID,null()),Month))

[agni_gold]

Thanks ,

Little bit change in requirement ,

Month	ID	Required_Output
Dec-15	3,2,1	null
Jan-16	3,4,8	null
Feb-16	2,3	3
Mar-16	9,2,4	null
Apr-16	6,1,5	null
May-16	6,5	null
Jun-16	6,5,4	null
Jul-16	5,4	5

to find common between strings type structure.