Why Doesn't My PostgreSQL WHERE Clause Recognize My SELECT Alias Column?

PostgreSQL: Addressing Alias Column Issues in WHERE Clauses

Encountering errors where PostgreSQL's WHERE clause fails to recognize SELECT aliases stems from the database's query processing order. Unlike some other systems (like MySQL), PostgreSQL processes the WHERE clause before the SELECT clause. Therefore, aliases defined only within the SELECT list are unavailable in the WHERE clause.

Effective Solution: Common Table Expressions (CTEs)

The most efficient solution is to employ a Common Table Expression (CTE). A CTE acts as a temporary, named result set, allowing you to define aliases and then reference them later in the query:

WITH jobs_refined 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_refined
WHERE lead_state = 'NEW';

This improved query uses the CTE jobs_refined to define lead_state. The WHERE clause can then correctly utilize this alias because the CTE has already processed and defined it. This approach avoids the original error and provides a cleaner, more readable query.

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