DAX Grouping and Ranking in Calculated Columns - powerbi

Question

My raw data stops at sales - looking for some DAX help adding the last two as calculated columns.
customer_id order_id order_date sales total_sales_by_customer total_sales_customer_rank
------------- ---------- ------------ ------- ------------------------- ---------------------------
BM 1 9/2/2014 476 550 1
BM 2 10/27/2016 25 550 1
BM 3 9/30/2014 49 550 1
RA 4 12/18/2017 47 525 3
RA 5 9/7/2017 478 525 3
RS 6 7/5/2015 5 5 other
JH 7 5/12/2017 6 6 other
AG 8 9/7/2015 7 7 other
SP 9 5/19/2017 26 546 2
SP 10 8/16/2015 520 546 2

Answer 1

Lets start with total sales by customer:
total_sales_by_customer =
var custID = orders[customer_id]
return CALCULATE(SUM(orders[sales], FILTER(orders, custID = orders[customer_id]))
first we get the custID, filter the orders table on this ID and sum it together per customer.
Next the ranking:
total_sales_customer_rank =
var rankMe = RANKX(orders, orders[total_sales_by_customer],,,Dense)
return if (rankMe > 3, "other", CONVERT(rankMe, STRING))
We get the rank per cust sales (gotten from first column), if it is bigger than 3, replace by "other"
On your first question: DAX is not like a programming language. Each row is assessed individual. Lets go with your first row: your custID will be "BM".
Next we calculate the sum of all the sales. We filter the whole table on the custID and sum this together. So in the filter we have actualty only 3 rows!
This is repeated for each row, seems slow but I only told this so you can understand the result you are getting back. In reality there is clever logic to return data fast.
What you want to do "Orders[Customer ID]=Orders[Customer ID]" is not possible because your Orders[Customer ID] is within the filter and will run with the rows..
var custid = VALUES(Orders[Customer ID]) Values is returning a single column table, you can not use this in a filter because you are then comparing a cell value with a table.