Find out who didn't pay in a monthly billing system - sql-server

Question

I'm creating system for manage shops rent. I have 4 tables (customers, shops, contracts, payments).
And this is the 4 tables:
customers :
CREATE TABLE [dbo].[customers](
[cust_id] [int] NOT NULL,
[name] [nvarchar](50) NULL,
[nickname] [nvarchar](50) NULL,
[city] [nvarchar](50) NULL,
[phone1] [nvarchar](15) NULL,
[phone2] [nvarchar](15) NULL,
[phone3] [nvarchar](15) NULL,
[email] [nvarchar](50) NULL,
[image] [varbinary](max) NULL,
[date] [date] NULL,
[image_exist] [nvarchar](5) NULL,
shops :
CREATE TABLE [dbo].[shops](
[shop_id] [int] NOT NULL,
[size] [nvarchar](5) NULL,
[floor] [nvarchar](10) NULL,
[location] [nvarchar](50) NULL,
[status] [nvarchar](10) NULL,
[date] [date] NULL,
Contracts:
CREATE TABLE [dbo].[contracts](
[con_id] [int] NOT NULL,
[cust_id] [int] NULL,
[shop_id] [int] NULL,
[con_duration] [nvarchar](5) NULL,
[price] [nvarchar](50) NULL,
[con_use] [nvarchar](20) NULL,
[rent_type] [nvarchar](10) NULL,
[price2] [nvarchar](50) NULL,
[note2] [nvarchar](max) NULL,
[image] [image] NULL,
[date_start] [date] NULL,
[date_end] [date] NULL,
[note] [nvarchar](max) NULL,
[image_exist] [nvarchar](5) NULL,
Payments:
CREATE TABLE [dbo].[payments_monthly](
[id] [int] IDENTITY(1,1) NOT NULL,
[con_id] [int] NULL,
[pay_number] [nvarchar](50) NULL, \\ i get the duration from contact and make numbers of payments, if the duration was 12 then the pay_number will be 1,2,3,.....,12- so the customers need to pay 12 time , one time per month
[pay_value] [decimal](18, 0) NULL, \\ amount of payment
[pay_type] [nvarchar](50) NULL,
[ch_number] [int] NULL,
[note] [nchar](10) NULL,
[date] [date] NULL,
So what I need is get all customers that didn't pay for this month or specific month the user chose (note : not all customers must pay in the same date, maybe some need to pay on 10-9-2017 or some 20-9-2017) so I need all customers who must pay in the September (1-9-2017 to 30-92017).
So what I need to change in my tables to achieve my goals, or query.
I am working on a program to manage a group of shops and these shops rented for a certain period, so the contract is 6 months, 12 months, 18 or 24 ... etc.
there is 2 type of contracts:
1-(The contract is a certain period and the payment is one time)
2-(the contract is a certain period and the payment per month in a certain amount per month)
For example:
1: 12000 $ for 12 months - paid once (stored in payment_yearly table)
Contract 2: 12000 $ for 12 months - paid more than once - the payment will be as follows (12000/12 = 1000 $per month) (stored in the payment_monthly table)
The program worked as follows:
Table of tenants
And a table for rented shops
A table of contracts (in which the customers and the shops premises are connected)
After that, 2 tables for monthly payments and yearly payments. Payments are recorded for each contract (linked to the customer and the shop)

Answer 1

What if they're late and pay two months at the same time, so on Sept 1 you get payments for August and September? A traditional accounting system would also have an Invoices table where each month a new invoice is created, and when a payment is made it's applied to the appropriate invoice. Then it's a simple matter of looking for open invoices.
Here's how I would create a simple invoice table:
CREATE TABLE invoices (
ID INT IDENTITY(1,1) PRIMARY KEY
, con_id INT FOREIGN KEY REFERENCES dbo.Invoices(con_id)
, invoice_date DATETIME NOT NULL DEFAULT GETDATE()
, payment_id INT FOREIGN KEY REFERENCES dbo.payments(id)
)
It references both the contract and the payment record. On the first of each month, run a process that automatically generates a new invoice for each active contract. Then, when a payment comes in, add the payment ID to the invoice so you know it's paid.
This is a fairly simple system, though, because it assumes only one payment can be applied to one invoice, which only makes sense if nobody ever makes a partial payment. A better option would be to create another table with a many-to-many relationship between payments and invoices:
CREATE TABLE invoice_payments (
ID INT IDENTITY(1,1) PRIMARY KEY
, invoice_id INT FOREIGN KEY REFERENCES dbo.invoices(ID)
, payment_id INT REFERENCES dbo.payments(id)
, DateCreated DATETIME NOT NULL DEFAULT GETDATE()
)
Now you can apply any number of payments to any number of invoices, and you can create a query that adds up all invoices and all payments to ensure that the client has paid all of their obligations!

Answer 2

I think the following example might give you some pointers
DECLARE #PaymentMonth INT
-- For Semptember 2017 set the variable to
SET #PaymentMonth = 201709
-- For March 2016 set the variable to
-- SET #PaymentMonth = 201603
SELECT c.Name
FROM customers cust
INNER JOIN contracts cont ON cust.cust_id = cont.cust_id
LEFT JOIN payments_monthly p ON cont.con_id = p.con_id
WHERE DATEPART(year, p.date) * 100 + DATEPART(month, p.date) <> #PaymentMonth