How to Use Aliases in PostgreSQL WHERE Clauses Without Errors?

Avoiding "column does not exist" Errors When Using Aliases in PostgreSQL WHERE Clauses

PostgreSQL queries can throw a "column does not exist" error when using aliases within the WHERE clause, even if the alias is defined in the SELECT list. This happens because the WHERE clause is processed before the SELECT list, meaning the alias isn't yet recognized.

To overcome this, use a WITH clause (also known as a Common Table Expression or CTE). A WITH clause creates a temporary named result set, allowing you to reference the alias within the main query.

Here's an example illustrating the solution:

WITH jobs_with_lead_state AS (
    SELECT
        jobs.*,
        CASE WHEN lead_informations.state IS NOT NULL THEN lead_informations.state ELSE 'NEW' END AS lead_state
    FROM jobs
    LEFT JOIN lead_informations
        ON lead_informations.job_id = jobs.id
        AND lead_informations.mechanic_id = 3
)
SELECT *
FROM jobs_with_lead_state
WHERE lead_state = 'NEW';

This revised query defines a CTE named jobs_with_lead_state. The CTE performs the initial SELECT and JOIN operations, generating the lead_state alias. The main query then uses this CTE, allowing the WHERE clause to correctly reference lead_state. This approach avoids the error by ensuring the alias is available during WHERE clause evaluation.

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