Why Does MySQL Require Aliases for Derived Tables?

Deciphering the "Every Derived Table Must Have Its Own Alias" Conundrum in MySQL

When encountering the error "Every derived table must have its own alias" in MySQL, it's crucial to delve into the concept of derived tables. These tables, also known as sub-queries, require an explicit alias to facilitate referencing within the outer query.

Delving into the Problem

Let's take the example query causing the error:

SELECT ID FROM (
    SELECT ID, msisdn
    FROM (
        SELECT * FROM TT2
    )
);

This query consists of two nested sub-queries. While the innermost sub-query retrieves data from the 'TT2' table, the error arises because both sub-queries lack an alias.

Crafting a Solution

To rectify this error, each sub-query must be assigned a unique alias. The corrected query would resemble:

SELECT ID FROM (
    SELECT ID, msisdn FROM (
        SELECT * FROM TT2
    ) AS T1
) AS T2

Here, both sub-queries are assigned aliases ('T1' and 'T2'), allowing the outer query to reference their respective results.

Streamlining the Query

In cases where the sub-query is simply extracting data from a table, the entire construct can be simplified.

SELECT ID FROM TT2

This streamlined query eliminates the sub-query layer while still achieving the desired result.

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