Can I Use a Calculated Column Alias in a SQL WHERE Clause?

SQL WHERE Clause and Calculated Column Aliases

Directly using a calculated column alias within a SQL WHERE clause is typically not supported. For example, this query will fail:

<code class="language-sql">SELECT (InvoiceTotal - PaymentTotal - CreditTotal) AS BalanceDue
FROM Invoices
WHERE BalanceDue > 0; -- Error</code>

Solutions:

Two effective methods circumvent this limitation:

1. Subquery (Nested Query):

This approach encapsulates the calculation within a subquery:

<code class="language-sql">SELECT BalanceDue
FROM (
  SELECT (InvoiceTotal - PaymentTotal - CreditTotal) AS BalanceDue
  FROM Invoices
) AS x
WHERE BalanceDue > 0;</code>

2. Expression Repetition:

Alternatively, replicate the calculation in both the SELECT and WHERE clauses:

<code class="language-sql">SELECT (InvoiceTotal - PaymentTotal - CreditTotal) AS BalanceDue
FROM Invoices
WHERE (InvoiceTotal - PaymentTotal - CreditTotal) > 0;</code>

Performance Considerations:

While the subquery might appear less efficient, modern database systems like SQL Server often optimize queries effectively. The execution plans for both methods are frequently identical, indicating similar performance.

Advanced Optimization: Computed Columns

For intricate calculations used repeatedly across multiple queries, a computed column offers a significant performance advantage. This pre-calculated column simplifies queries and reduces redundant computations.

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