How can I calculate data from different tables?

[saranp780]

Hello. I'm really new to Power BI. I want to know about "How can I calculate data from different tables?".

If I had data like this

Table A 

Position	Username	Year	Counter	Peak
Manager	ACC001	2017	3	14
Acc. staff	ACC002	2017	20	15

This table maintains in Aug.2017.

Table B

Position	Username	Year	Counter	Peak
Manager	ACC001	2017	4	5
Acc. staff	ACC002	2017	25	11

 This table maintains in Sep.2017

My question is I want to subtract column "counter" table b(counter)-table a(counter).

So how the formula should I fill in? (Power BI desktop) 

[fhill]

Is Username a unique value in each table?  (Would it appear twice if there was multiple years of data?)  We need something to logically link 'Relationship' the two tables together.  

[saranp780]

Could you please explain an unique value? If I have to change my data, how to change?

Best regard

[fhill]

Here's some information on defining Relationships between tables:

https://powerbi.microsoft.com/en-us/documentation/powerbi-desktop-create-and-manage-relationships/