How Can I Define Named Constants in PostgreSQL Queries?

PostgreSQL: Working with Named Constants in Queries

Unlike some programming languages, PostgreSQL doesn't support directly defining named constants within a SQL query. The following example won't work:

<code class="language-sql">MY_ID = 5;
SELECT * FROM users WHERE id = MY_ID;</code>

Effective Solution: Common Table Expressions (CTEs)

The most effective workaround involves using Common Table Expressions (CTEs). A CTE lets you define a named constant for internal use within the query:

<code class="language-sql">WITH my_constants AS (
    SELECT 5 AS my_id
)
SELECT *
FROM users, my_constants
WHERE users.id = my_constants.my_id;</code>

This approach defines my_id as a constant within the my_constants CTE. The main query then joins with this CTE, making the constant accessible for use in the WHERE clause.

This CTE method proves especially useful when dealing with constant date values or other parameters across multiple subqueries. It ensures consistency and improves readability.

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