Why Does Mixing JOINs and Commas in MySQL Queries Cause 'Unknown column' Errors?

Mixing JOIN and commas in MySQL queries causes errors

Problem Description

When executing the following MySQL 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>

will encounter the following error:

<code>#1054 - Unknown column 'm.id' in 'on clause'</code>

The query fails even though the column exists. This error occurs when ANSI 1992 JOIN syntax and commas are used together in a query.

Solution

According to the MySQL documentation, mixing JOINs and commas should be avoided. To resolve this issue, replace the comma with the appropriate JOIN operator (CROSS, INNER, LEFT).

The updated query using INNER JOIN is as follows:

<code class="language-sql">SELECT m.*, t.*
FROM memebers as m
INNER JOIN telephone as t ON <join condition>
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>

Note that the join condition between m and t should be specified to avoid Cartesian product.

Additional Notes

For clarity and efficiency, the JOIN operator should be used instead of commas. The comma indicates a CROSS JOIN, which may cause unexpected behavior, especially when combining multiple tables. By using explicit JOIN operators, you can define specific relationships between tables to ensure you get the results you want.

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