¿How to calculate a difference between rows in a select in SQL Oracle?

Question

I would like your help for a Oracle sql construction.

Let's say I have below database tables and descriptions: Store: store the information related to all new customers country allows to take two values: “Argentina” / “Brasil”

CustomerBOM: stores historical clientes Date: First date of each month (1/01/2017, 1/02/2017…) Client: a customer who's subscription is up to date.

Now I have to answer this question: Which is the quantity for each month?

The code I've made so far is shown below:

select date,  
from CustomerBOM t1, Store t2
where t1.ID = t2.ID
group by date
having 
order by date asc

Can you please guide me on how to have a full list with the difference between each month?

Edit: This is how it should look like the output of the sentence:

Month         Difference 
January       100 (total clients) 
February      20 (120 clients from February - 100 clients from January)
March         60 (180 clients from March - 120 clients from February)

Thanks, Nicolás.

Answer 1

Try this:

select to_char(t1.date, 'YYYY/MM/DD') customer_month,  count(1) number_of_customers
from CustomerBOM t1, 
     Store t2
where t1.ID = t2.ID
group by to_char(t1.date, 'YYYY/MM/DD')
order by to_char(t1.date, 'YYYY/MM/DD')