For Loop Count Unique

Question

I have a table like this

month	customer_id
2015-01-01	customer_A
2015-01-01	customer_B
2015-02-01	customer_A
2015-02-01	customer_C
2015-03-01	customer_A
2015-03-01	customer_C
2015-03-01	customer_D
2015-04-01	customer_C
2015-04-01	customer_D

I want to find out how active my customers are every month.

I tried to write a loop that can do like the example below:

1. Filter to count the unique customer_id in Jan 2015 (in the table above: 2)

2. Filter all the customer_id in Feb 2015 and check how many are in the customer list of Jan 2015. (in the table above: 1)

3. Continue to the last month

4. Loop back to start from Feb 2015

The result should be a table like this (the numbers are random)

month	month_1	customers
2015-01-01	2015-02-01	2 (A&B)
2015-01-01	2015-02-01	1 (only A left)
2015-01-01	2015-03-01	1 (only A left)
2015-01-01	2015-04-01	0 (none in the original left)
2015-02-01	2015-03-01	2 (A&C)
2015-02-01	2015-04-01	1 (only C left)

Thank you so much!