Question

I’m trying to join 4 tables, two 1-N ones and one through an N-N table. Strangely mySql doesn’t seem to like one of my syntaxes. Does anybody know if this is due to myOwn limitations or mySql?

This doesn’t work:

SELECT *
FROM tableOne t1 JOIN tableTwo t2 
    LEFT OUTER JOIN N_N_tableThree t3 
            JOIN tableFour t4 ON t4.id = t3.fk_tableFour
        ON t2.id = t3.fk_tableTwo
    ON t2.id = t1.fk_tableTwo

While this does work

SELECT *
FROM tableOne t1,
tableTwo t2 LEFT OUTER JOIN N_N_tableThree t3 
        JOIN tableFour t4 ON t4.id = t3.tableFour_id
    ON t2.id = t3.tableTwo_id
WHERE t2.id = t1.tableTwo_id

Anybody a clue?

Thanks for answering.

Answer 1

Use this syntax instead:

SELECT *
FROM tableOne            t1 
INNER JOIN tableTwo      t2 ON t2.id = t1.fk_tableTwo
LEFT JOIN N_N_tableThree t3 ON t2.id = t3.fk_tableTwo
INNER JOIN tableFour     t4 ON t4.id = t3.fk_tableFour;

This will be equivalent of the second query that worked.

Because the WHERE t2.id = t1.tableTwo_id in the second query is actually an INNER JOIN¹, which will be the same as INNER JOIN tableTwo t2 ON t2.id = t1.fk_tableTwo as I did. This is the old JOIN syntax, try to avoid it and use the ANSI SQL-92 syntax instead as I did. For more information see this:

• Bad habits to kick : using old-style JOINs.

The query you posted didn’t work, because it is not the correct syntax for JOIN in MySQL, you have three ON clauses after each others:

...
ON t4.id = t3.fk_tableFour
ON t2.id = t3.fk_tableTwo
ON t2.id = t1.fk_tableTwo

Each JOIN should has the join condition specified with the ON clause after the JOIN directly, if not it would a cross join². But not multiple ON‘s the way you did.

SQL Fiddle Demo

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^{1: Don’t be confused with the use of INNER JOIN instead of JOIN they are the same the default JOIN is inner join, I just I used for readability. Also, the same thing with the OUTER keyword, I omit it in LEFT JOIN since it is optional when using LEFT or RIGHT}

^{2: You will find other variations of the JOIN syntax in MySQL in the reference page, like the JOIN tablename without a join condition, and others. You might need to read them.}