Why Can't I Use a Calculated Alias in My SQL WHERE Clause?

SQL WHERE Clause and Calculated Aliases: A Common Problem

SQL allows creating calculated aliases within SELECT statements. However, directly referencing these aliases in the WHERE clause often results in an error. This limitation can be problematic when working with intricate calculations.

Effective Solutions

Several approaches circumvent this restriction:

1. Employing Subqueries:

   • Nest the calculation within a subquery, assigning a name to the calculated alias. Then, reference this name in the outer query's WHERE clause.

   • Illustrative Example:

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

2. Repeating the Calculation:

   • Instead of using the alias, replicate the calculation directly within the WHERE clause.
   • Illustrative Example:

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

Performance Implications

While subqueries are straightforward, they can lead to performance degradation. Conversely, repeatedly complex calculations within the WHERE clause can also negatively impact performance.

Optimal Strategy

For simple calculations, repeating the calculation is generally more efficient. For complex computations, creating a persistent computed column is recommended. This pre-calculated column avoids redundant calculations and ensures data consistency across queries.

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