MySQL calculate gain, loss and net gain over a period of time - mysql

Question

I have a table something like this:
id | Customer | date
-----------------------------------------
1 | Customer2 | 2013-08-01 00:00:00
-----------------------------------------
2 | Customer1 | 2013-07-15 00:00:00
-----------------------------------------
3 | Customer1 | 2013-07-01 00:00:00
-----------------------------------------
. | ... | ...
-----------------------------------------
n | CustomerN | 2012-03-01 00:00:00
I want to calculate the "gained" customers for each month, the "lost" customers for each month and the Net Gain for each month, even if done in separate tables / views.
How can I do that?
EDIT
Ok, let me demonstrate what I've done so far.
To select Gained customers for any month, I've tried to select customers from Bookings table where the following not exist:
select Customer
from Bookings
where not exists
(select Customer
from Bookings
where
(Bookings.date BETWEEN
DATE_FORMAT(DATE_SUB(Bookings.date, INTERVAL 1 MONTH), '%Y-%m-01 00:00:00')
AND DATE_FORMAT(Bookings.date, '%Y-%m-01 00:00:00'
)
) AND Bookings.date >= STR_TO_DATE('2010-11-01 00:00:00', '%Y-%m-%d 00:00:00'))
This supposedly gets the customers that existed in the "selected" month but not in the previous one. "2010-11-01" is the date of the start of bookings + 1 month.
To select Lost customers for any month, I've tried to select customers from Bookings table where the following not exist:
select Customer
from Booking
where not exists
(select Customer
from Bookings
where
(Bookings.date BETWEEN
DATE_FORMAT(Bookings.date, '%Y-%m-01 00:00:00')
AND Bookings.date
)
AND Bookings.date >= STR_TO_DATE('2010-11-01 00:00:00', '%Y-%m-%d 00:00:00'
)
)
This supposedly gets the customers that existed in a previous month but not in the "selected" one.
For the "Loss" SQL query I got empty result! For the "Gain" I got thousands of rows but not sure if that's accurate.

Answer 1

You can use COUNT DISTINCT to count your customers, and WHERE YEAR(Date) = [year] AND MONTH(Date) = [month] to get the month.
The total number of customers in Sept 2013:
SELECT COUNT(DISTINCT Customer) AS MonthTotalCustomers FROM table
WHERE YEAR(date) = 2013 AND MONTH(date) = 9
The customers gained in Sept 2013:
SELECT COUNT(DISTINCT Customer) AS MonthGainedCustomers FROM table
WHERE YEAR(date) = 2013 AND MONTH(date) = 9
AND Customer NOT IN
(SELECT Customer FROM table
WHERE date < '2013-09-01')
Figuring out the lost customers is more difficult. I would need to know by what criteria you consider them to be 'lost.' If you just mean that they were around in August 2013 but they were not around in September 2013:
SELECT COUNT(DISTINCT Customer) AS MonthLostCustomers FROM table
WHERE YEAR(date) = 2013 AND MONTH(date) = 8
AND Customer NOT IN
(SELECT Customer FROM table
WHERE YEAR(date) = 2013 AND MONTH(date) = 9)
I hope from these examples you can extrapolate what you're looking for.