How Can I Efficiently Compare Nullable Variables in SQL Server Queries?

Optimizing NULL Comparisons in SQL Server

When dealing with nullable variables in a SQL Server query, it's important to handle comparisons efficiently. Suppose you have a variable @OrderID that might be NULL. To query data based on the variable's value, you may resort to using IF ELSE statements to check for NULL. However, there is a more concise way to perform such comparisons in a single query.

The recommended approach is to utilize the EXISTS operator with the INTERSECT set operator. The following code snippet demonstrates this:

SELECT * 
FROM Customers
WHERE EXISTS (SELECT OrderID INTERSECT SELECT @OrderID);

This query will efficiently check if the value of @OrderID exists in the OrderID column of the Customers table. If @OrderID is NULL, it will return rows where OrderID is also NULL. Conversely, if @OrderID has a non-NULL value, it will return rows where OrderID equals @OrderID.

This approach adheres to the "Undocumented Query Plans: Equality Comparisons" principle, which suggests that optimized plans are generated when comparing a variable to itself using INTERSECT. By employing this technique, you can streamline your queries and improve performance when dealing with nullable variables.

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