Why Does Mixing ANSI JOINs and Comma-Separated Tables in MySQL Lead to 'Unknown Column' Errors?

Problems using mixed ANSI JOIN and comma separated tables in MySQL queries

You may encounter an "Unknown column" error when mixing ANSI JOIN syntax (INNER JOIN, CROSS JOIN, LEFT JOIN) and comma-separated table references in a MySQL query. This error occurs because MySQL's interpretation of the comma operator depends on its position.

Question:

Consider the following query:

<code class="language-sql">SELECT m.*, t.*
FROM memebers as m, telephone as t
INNER JOIN memeberFunctions as mf ON m.id = mf.memeber
INNER JOIN mitgliedTelephone as mt ON m.id = mt.memeber
WHERE mf.function = 32</code>

This query results in an "Unknown column 'm.id' in 'on clause'" error.

Solution:

To resolve this error, avoid mixing ANSI JOIN syntax with comma-separated tables. Use only ANSI JOIN syntax instead, as shown in the modified query below:

<code class="language-sql">SELECT m.*, t.*
FROM memebers as m
JOIN telephone as t ON 1=1
INNER JOIN memeberFunctions as mf ON m.id = mf.memeber AND mf.function = 32
INNER JOIN mitgliedTelephone as mt ON m.id = mt.memeber</code>

Explanation:

MySQL now interprets the query as:

<code class="language-sql">((memembers as m JOIN telephone as t ON 1=1) INNER JOIN memeberFunctions as mf ON m.id = mf.memeber) INNER JOIN mitgliedTelephone as mt ON m.id = mt.memeber</code>

In this syntax, the operands of the JOIN clause are clearly defined and the "m.id" column is correctly referenced in the ON clause. ON 1=1 is added to ensure the correctness of the JOIN operation, because the JOIN keyword requires the ON clause.

Note:

It is important to note that using commas to separate tables is no longer considered a best practice in MySQL. It is recommended to use ANSI JOIN syntax to ensure clarity and avoid potential errors.

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