Query to Sum Values from One Table Based on Values from Another Table, Considering Null Values

In this article, we’ll explore a common database query problem where you need to calculate the remaining value for each contract based on payments made. We’ll break down the steps involved in solving this problem and provide examples using MS Access.

Understanding the Problem

The problem involves two tables: Contracts and Payments. The Contracts table contains information about contracts, including the contract ID and total value. The Payments table contains records of payments made for each contract, with the payment amount and contract ID.

We want to write a query that calculates the remaining value for each contract based on the payments made. If there are no payments for a particular contract, we should show the total value of the contract instead of zero.

Query 1: Summing Payments

First, let’s analyze the given queries:

Query 1:

SELECT Contracts.[Contract ID],
       SUM(Payments.[Payment Value]) AS SumOfValue Payment Value,
FROM Contracts 
LEFT JOIN Payments 
ON Contracts.[Contract ID] = Payments.[Contract ID]
GROUP BY Contracts.[Contract ID]

This query joins the Contracts table with the Payments table using a LEFT JOIN. It then groups the results by contract ID and calculates the sum of payment values for each contract.

However, this query has a limitation: if there are no payments for a particular contract (i.e., NULL values in the Payment Value column), it will result in a zero value for that contract.

Query 2: Calculating Net Values

Query 2:

SELECT Contracts.[Contract ID], 
       Contracts.[Contract Value], 
       Query1.[SumOfValue Payment Value], 
       [Contract Value] - [SumOfValue Payment Value] AS Remaining
FROM Contracts
INNER JOIN Query1 ON Contracts.[Contract ID] = Query1.[Contract ID]

This query joins the Contracts table with Query 1, which sums the payment values. It then calculates the remaining value for each contract by subtracting the sum of payment values from the total value.

Limitation and Solution

However, if there are no payments for a particular contract (i.e., NULL values in the SumOfValue Payment Value column), Query 2 will also result in zero values for that contract. To solve this problem, we need to modify our approach to handle NULL values correctly.

The Answer: A Single Query

Here’s an example of how you can write a single query to calculate the remaining value for each contract based on payments made:

SELECT   c.[Contract ID], 
         c.[contract value], 
         p.Paid, 
         c.[contract value]-nz(p.paid) AS Remaining
FROM     Contracts AS c 
  LEFT JOIN 
         (SELECT [Contract ID], sum([Payment Value]) as paid
          FROM Payments
          group by [Contract ID])  AS p 
    ON    c.[Contract ID] = P.[Contract ID];

In this query:

1. We join the Contracts table with a derived table, which sums the payment values for each contract.
2. We use a LEFT JOIN to include contracts without payments (i.e., NULL values in the Paid column).
3. We calculate the remaining value for each contract by subtracting the paid amount from the total value.

Handling NULL Values

To handle NULL values correctly, we use the nz() function in MS Access. This function returns the first non-NULL value in a specified range.

The query will produce this result:

As you can see, the query correctly calculates the remaining value for each contract based on payments made.

Conclusion

In this article, we explored a common database query problem where we need to calculate the remaining value for each contract based on payments made. We analyzed two given queries and identified their limitations. Finally, we provided an example of how to write a single query using MS Access to solve this problem.

By following these steps and understanding how to handle NULL values correctly, you can solve similar database query problems in your own work.

Last modified on 2024-09-01