Why Does My PostgreSQL WHERE Clause Fail to Recognize an Aliased Column, and How Can I Fix It Using a CTE?

PostgreSQL WHERE Clause and Aliased Columns: A Problem and its Solution

When using an aliased column (like "lead_state") within a PostgreSQL WHERE clause, you might encounter an error indicating the column doesn't exist. This is because PostgreSQL processes the WHERE clause before the SELECT clause, unlike MySQL. This means the alias isn't yet defined when the WHERE clause is evaluated.

Solving the Issue with a Common Table Expression (CTE)

A robust solution is to utilize a Common Table Expression (CTE). A CTE acts as a temporary, named result set within your query. This allows you to define the aliased column within the CTE and then reference it in the main query's WHERE clause.

Here's how you can implement this:

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';

The CTE, "jobs_refined," generates the "lead_state" column. The subsequent SELECT statement then uses this CTE, making "lead_state" available for the WHERE clause, effectively resolving the original error. This approach ensures correct query execution in PostgreSQL.

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