calculates between These two columns in SQL server

I want to add a column to the query that calculates between These two columns In the same query ....................................................................

  ,isnull(sum(ORDERS.Net_Amount) + 0,0) as orders
  ,isnull(sum (convert(float,(RECEIPTS.Amount))) + 0,0) as recepts

IN SQL SERVER

SELECT CUSTOMERS.[ID_CUSTOMER]
      ,[FIRST_NAME]
      ,[TEL]
      ,[EMAIL]
      ,isnull(sum(ORDERS.Net_Amount) + 0,0) as orders
      ,isnull(sum (convert(float,(RECEIPTS.Amount))) + 0,0) as recepts
      ,[CRIDIT_LIMIT]
      ,[CUSTOMER_SINCE]
      ,[ADRESS]
      ,CUSTOMERS.[state]
  FROM [CUSTOMERS]
LEFT JOIN ORDERS on CUSTOMERS.ID_CUSTOMER = ORDERS.CUSTOMER_ID
LEFT JOIN RECEIPTS on CUSTOMERS.ID_CUSTOMER = RECEIPTS.ID_CUSTOMER
GROUP BY CUSTOMERS.[ID_CUSTOMER]
      ,[FIRST_NAME]
      ,[TEL]
      ,[EMAIL]
      ,[CRIDIT_LIMIT]
      ,[CUSTOMER_SINCE]
      ,[ADRESS]
      ,CUSTOMERS.[state]

CUSTOMERS table

SELECT [ID_CUSTOMER]
      ,[FIRST_NAME]
      ,[TEL]
      ,[EMAIL]
      ,[IMAGE_CUSTOMER]
      ,[CRIDIT_LIMIT]
      ,[CUSTOMER_SINCE]
      ,[ADRESS]
      ,[Balance]
      ,[state]
  FROM [CUSTOMERS]

ORDERS table

SELECT [ID_ORDER]
      ,[DATE_ORDER]
      ,[CUSTOMER_ID]
      ,[DESCRIPTION_ORDERS]
      ,[SALEMAN]
      ,[ORDER_TOTAL]
      ,[Discount_Of_Total]
      ,[Total_After_Discount]
      ,[Paid_Up]
      ,[Net_Amount]
      ,[state]
  FROM [ORDERS]

RECEIPTS table

SELECT [image_state]
      ,[ID_RECEIPT]
      ,[ID_CUSTOMER]
      ,[Date]
      ,[Ref]
      ,[Amount]
      ,[Memo]
      ,[User_Name]
      ,[state]
      ,[Payment_Method]
      ,[Account_ID]
  FROM [RECEIPTS]

Solution

Orders and receipts are not really related to each other (the receipt doesn't refer to a specific order), so don't join the two. What you want to do instead is find the order amount and the receipt amount per customer and show them. So aggregate the two tables per customer and outer-join the results to the customer table.

 select
   c.id_customer,
   c.first_name,
   c.tel,
   c.email,
   coalesce(o.sum_net_amount, 0) as order_amount,
   coalesce(r.sum_amount, 0) as receipt_amount,
   c.cridit_limit,
   c.customer_since,
   c.adress,
   c.balance,
   c.state
 from customers c
 left join
 (
   select customer_id, sum(net_amount) as sum_net_amount
   from orders
   group by customer_id
 ) o on c.id_customer = o.customer_id
 left join
 (
   select id_customer, sum(amount) as sum_amount
   from receipts
   group by id_customer
 ) r on c.id_customer = r.id_customer;

I see you have updated your request now asking also for the difference of the sums. Well, the operator for subtraction in SQL is - little surprising - the minus sign:

   coalesce(o.sum_net_amount, 0) - coalesce(r.sum_amount, 0) as diff