Have you ever heard the advice “Never place a CASE expression in a JOIN clause” when working with SQL Server? While this is a common best practice, it’s important to understand the reasons behind it rather than blindly following it. In this tutorial, we will explore the many uses of a CASE expression and discuss why you might consider using it in a JOIN clause.

Exploring SQL CASE Expressions

A CASE expression in SQL Server allows you to evaluate a list of conditions and return one of multiple possible result expressions. It is a powerful tool that can be used in various parts of a SQL statement, including the SELECT, WHERE, and JOIN clauses.

Here is an example of a simple CASE expression:

SELECT CASE
           WHEN 1 = 1 THEN
               1
           WHEN 2 = 2 THEN
               2
           WHEN 3 = 3 THEN
               3
           ELSE
               0
       END AS [My Favorite Number];

In this example, the CASE expression evaluates the conditions and returns the corresponding result expression. In this case, the result will be 1 because the condition “1 = 1” is true.

You can also nest CASE expressions for added control, although it is recommended to keep the code readable and avoid excessive nesting.

Using CASE Expressions in JOIN Clauses

One of the places where you can use a CASE expression is in a JOIN clause. This can be useful when you have complex join logic that requires joining a table based on multiple conditions.

For example, let’s say you have two tables: Person and Sales. You primarily want to join these tables based on the SalesPersonId, but in some cases, you want to join on the AccountManagerId when the OrderCode has a specific value.

SELECT CONCAT(p.LastName, ', ', p.FirstName) AS PersonName,
       SUM(s.Price) AS TotalSales,
       s.SalesDate
FROM dbo.Sales s
    INNER JOIN dbo.Person p
        ON p.Id = CASE
                      WHEN s.OrderCode = 'ABC' AND s.AccountManagerId IS NOT NULL THEN
                          s.AccountManagerId
                      ELSE
                          s.SalesPersonId
                  END
GROUP BY s.SalesDate, p.FirstName, p.LastName;

In this example, the CASE expression is used in the JOIN clause to determine the join condition based on the values of OrderCode and AccountManagerId. This allows you to join the tables based on different conditions.

Another use case for using a CASE expression in a JOIN clause is to handle NULL values returned from one of the tables. This can be useful when an ISNULL() function doesn’t provide the desired results.

Alternative Approaches

While using a CASE expression in a JOIN clause can be a valid approach, there are alternative methods that you can consider. One commonly recommended alternative is using the UNION ALL operator.

Here is an example of how you can achieve the same results without using a CASE expression:

SELECT CONCAT(p.LastName, ', ', p.FirstName) AS PersonName,
       SUM(s1.Price) AS TotalSales,
       s1.SalesDate
FROM dbo.Sales s1
    INNER JOIN dbo.Person p
        ON p.Id = s1.SalesPersonId
WHERE s1.OrderCode <> 'ABC' OR s1.AccountManagerId IS NULL
GROUP BY s1.SalesDate, p.FirstName, p.LastName
UNION ALL
SELECT CONCAT(p.LastName, ', ', p.FirstName) AS PersonName,
       SUM(s2.Price) AS TotalSales,
       s2.SalesDate
FROM dbo.Sales s2
    INNER JOIN dbo.Person p
        ON p.Id = s2.AccountManagerId
WHERE s2.OrderCode = 'ABC' AND s2.AccountManagerId IS NOT NULL
GROUP BY s2.SalesDate, p.FirstName, p.LastName;

Using the UNION ALL operator, you can create two separate datasets that match the join criteria and then combine them. This approach can be an alternative to using a CASE expression in certain scenarios.

Conclusion

Using a CASE expression in a JOIN clause can be a powerful technique to implement complex join logic and handle different conditions. While it is important to consider alternative approaches and performance implications, it is not always a bad practice as some may suggest.

Next time you come across the advice to never use a CASE expression in a JOIN clause, ask for an explanation and consider the specific requirements of your query. CASE expressions can be a valuable tool in your SQL Server toolbox.

Do you ever use a CASE expression in a JOIN clause? Let us know in the comments below!

Article Last Updated: 2023-05-17