How to Correctly GROUP BY Aliased Columns in SQL Server?

Group by alias column in SQL Server

Performing GROUP BY operations on aliased columns can be tricky, but understanding the underlying syntax is crucial. Let’s explore the right approach.

In your example, the alias column is 'FullName', which is created using the expression 'LastName ', ' FirstName'. To group by this alias, you cannot simply use the alias like "GROUP BY 'FullName'". Instead, you need to pass the actual expression that creates the alias.

The correct syntax is:

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

By specifying an expression, you can instruct SQL Server to perform grouping based on the calculated value of 'FullName', ensuring accurate results.

Expanding further, if the alias column is exported using a CASE statement, such as in your second example, the same principle applies. The GROUP BY clause should still contain the expression that forms the alias column, in this case:

SELECT CASE
    WHEN LastName IS NULL THEN FirstName
    WHEN LastName IS NOT NULL THEN LastName + ', ' + FirstName
END AS 'FullName'
FROM customers
GROUP BY CASE
    WHEN LastName IS NULL THEN FirstName
    WHEN LastName IS NOT NULL THEN LastName + ', ' + FirstName
END

Remember, when grouping by an alias column, always use the expression that creates the alias in the GROUP BY clause to ensure proper aggregation of the data.

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