How to Use Aliases in Postgresql WHERE Clauses?

Solving the Postgresql Alias in WHERE Clause Issue

Postgresql queries sometimes throw an error indicating a non-existent column when using aliases within the WHERE clause. This happens because the WHERE clause's evaluation precedes the SELECT clause, making aliases defined in SELECT unavailable.

The solution involves using Common Table Expressions (CTEs). A CTE creates a temporary, named result set that can be referenced later in the query, including within the WHERE clause. This effectively allows alias usage in the WHERE condition. Here's an illustration:

<code class="language-sql">WITH job_details AS (
    SELECT
        jobs.*, 
        CASE
            WHEN lead_informations.state IS NOT NULL THEN lead_informations.state
            ELSE 'NEW'
        END AS lead_status
    FROM
        jobs
        LEFT JOIN lead_informations ON
            lead_informations.job_id = jobs.id
            AND
            lead_informations.mechanic_id = 3
)
SELECT *
FROM job_details
WHERE lead_status = 'NEW';</code>

The CTE, "job_details," generates a temporary table including the aliased column "lead_status." The WHERE clause then seamlessly uses this alias as if referencing a standard table column. This approach enhances query flexibility and readability.

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