How Do I Use GROUP BY with Aliased Columns and CASE Statements in SQL Server?

SQL Server GROUP BY: Aliases and CASE Statements

This guide clarifies the correct syntax for using GROUP BY with aliased columns and CASE statements in SQL Server. Understanding this is crucial for accurate data aggregation.

Grouping by Aliases

Directly using an alias in the GROUP BY clause is incorrect. Instead, you must repeat the expression used to define the alias. For example, to group by a calculated FullName:

SELECT LastName + ', ' + FirstName AS FullName
FROM customers
GROUP BY LastName + ', ' + FirstName;

Grouping with CASE Statements

The same principle applies when your alias is derived from a CASE statement. The GROUP BY clause must mirror the CASE expression exactly:

SELECT       
    CASE
        WHEN LastName IS NULL THEN FirstName
        ELSE LastName + ', ' + FirstName
    END AS FullName
FROM         customers
GROUP BY     
    CASE
        WHEN LastName IS NULL THEN FirstName
        ELSE LastName + ', ' + FirstName
    END;

Remember: The expression in GROUP BY must precisely match the expression creating the alias to ensure correct grouping of identical values. This ensures data integrity when aggregating results.

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