Question

I have the following MySQL query:

SELECT posts.id,  posts.name,  LEFT(posts.content, 400),  posts.author,  posts.date, users.display_name,
  GROUP_CONCAT(tags.tag ORDER BY tag_linking.pid ASC SEPARATOR ",") update_tags
FROM posts, tag_linking, tags
INNER JOIN `users`
ON posts.author=users.id;
WHERE tag_linking.pid = posts.id 
  AND tags.id = tag_linking.tid 
ORDER BY posts.date DESC

Which, was you can see, connects three tables etc. etc. Anyway, the problem is that it gives an error:

ERROR CODE:
SQL Error (1054): Unknown column 'posts.author' in 'on clause'

even though this simpler query used on another page works:

SELECT posts.id,  posts.name,  LEFT(posts.content, 400),  posts.author,  posts.date, users.display_name FROM `posts`
INNER JOIN `users`
ON posts.author=users.id

Does anyone have thoughts as to why this is occuring? Thanks for your help.

Answer 1

because your mix join syntax

From Mysql^[docs]

However, the precedence of the comma operator is less than of INNER
JOIN, CROSS JOIN, LEFT JOIN, and so on. If you mix comma joins with
the other join types when there is a join condition, an error of the
form Unknown column ‘col_name’ in ‘on clause’ may occur. Information
about dealing with this problem is given later in this section.

the solution is:

To allow the join to be processed, group the first two tables
explicitly with parentheses so that the operands for the ON clause are
(t1,t2) and t3:

SELECT * FROM (t1, t2) JOIN t3 ON (t1.i1 = t3.i3);

Alternatively, avoid the use of the comma operator and use JOIN
instead:

SELECT * FROM t1 JOIN t2 JOIN t3 ON (t1.i1 = t3.i3);