Why Does MySQL Throw a '#1054 - Unknown Column' Error When Using Column Aliases in WHERE Clauses?

MySQL WHERE Clause and Column Aliases: Troubleshooting the "#1054 - Unknown column" Error

MySQL queries sometimes produce the error "#1054 - Unknown column 'guaranteed_postcode' in 'IN/ALL/ANY subquery'" when using column aliases within the WHERE clause. This is because standard SQL prevents the use of aliases in the WHERE clause due to potential inconsistencies in column value determination during execution. The database hasn't yet assigned values to the alias when the WHERE clause is processed.

The MySQL manual clearly states that aliases are only valid in GROUP BY, ORDER BY, or HAVING clauses. These clauses operate after data retrieval, guaranteeing alias availability.

Your query likely uses SUBSTRING(locations.raw,-6,4) aliased as guaranteed_postcode in the WHERE clause. Because MySQL encounters the alias before the column value is calculated, it generates the error.

To correct this, use the HAVING clause (for aggregate functions or calculations) or a subquery within the WHERE clause. The subquery avoids using the alias directly in the main query's WHERE clause, providing the necessary filtering without violating MySQL's rules.

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