How Can I Use Calculated Columns in PostgreSQL Queries Without Errors?

PostgreSQL Calculated Columns: Workarounds for Limitations

PostgreSQL's handling of calculated columns differs from other database systems. Directly referencing calculated columns later in the same query isn't supported. This limitation is demonstrated by the following example, which will produce an error:

<code class="language-sql">SELECT cost_1, quantity_1, cost_2, quantity_2, 
       (cost_1 * quantity_1) AS total_1,
       (cost_2 * quantity_2) AS total_2,
       (total_1 + total_2) AS total_3  -- Error: total_1 and total_2 are not found
FROM data;</code>

The error arises because PostgreSQL processes the query row-by-row; total_1 and total_2 aren't available for subsequent calculations within the same SELECT statement.

The solution involves using a subquery (derived table) to encapsulate the initial calculations. This allows the results to be referenced in the outer query:

<code class="language-sql">SELECT cost_1, quantity_1, cost_2, quantity_2, total_1, total_2,
       total_1 + total_2 AS total_3
FROM (
    SELECT cost_1, quantity_1, cost_2, quantity_2, 
           (cost_1 * quantity_1) AS total_1,
           (cost_2 * quantity_2) AS total_2
    FROM data
) AS t;</code>

By nesting the calculations of total_1 and total_2 within the inner SELECT statement (aliased as t), they become accessible and usable for further calculations in the outer SELECT statement. This method provides a practical and efficient way to work with calculated columns in PostgreSQL without significant performance overhead.

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