Why Does MySQL Require Aliases for All Derived Tables?

Understanding the Error: "Every Derived Table Must Have Its Own Alias"

When executing a MySQL query involving nested derived tables (sub-queries), an error may occur stating "Every derived table must have its own alias." This error is triggered when sub-queries are not assigned unique aliases.

Cause and Solution:

A derived table is created by enclosing a sub-query in parentheses. For each derived table, MySQL requires the inclusion of an alias, which is a name used to refer to the table and its data. The purpose of the alias is to distinguish among multiple derived tables within the outer query.

To resolve this error, assign an alias to each sub-query using the AS keyword followed by a unique name:

SELECT ID FROM (
    SELECT ID, msisdn FROM (
        SELECT * FROM TT2
    ) AS TT2Subquery
) AS TableAlias

In this modified query, the alias TT2Subquery is assigned to the innermost sub-query, and TableAlias is assigned to the outer sub-query. These aliases allow MySQL to differentiate between the two sub-queries and connect them appropriately.

Alternatively, since the outermost sub-query only retrieves a single column, it can be omitted entirely:

SELECT ID FROM TT2

This simplified query eliminates the need for any sub-query aliases while maintaining the same result.

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