How Can I Correctly Reference Aliases in a SQL WHERE Clause?

Referencing Aliases in WHERE Clause

When working with SQL queries, it is often necessary to use aliases to give temporary names to tables or expressions. This can improve the readability and maintainability of your code. However, it is important to be aware of the limitations when referencing aliases in different parts of the query.

The Issue

In the example provided, an attempt is made to reference an alias (_year) in the WHERE clause of a query. However, this results in an error:

ERROR:  column "_year" does not exist

This error occurs because the WHERE clause is executed first, before the SELECT clause. At the time of execution, the database has not yet encountered the alias definition in the SELECT clause and therefore cannot recognize it.

Solution

To work around this issue, you can rewrite the query so that the alias is defined in the WHERE clause itself. This ensures that the alias is available when the WHERE clause is executed:

SELECT
    SUBSTRING(pk, 6, 2)::INT AS _year
FROM
    listing
WHERE
    SUBSTRING(pk, 6, 2)::INT > 90

By placing the alias definition in the WHERE clause, you can reference the alias within the same clause, without encountering errors.

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