Question

What’s the difference between these two queries:

SELECT `threads`.`id` AS `threads.id` , `posts`.`id` , `posts`.`content`
FROM `threads`
JOIN `posts` ON `threads`.`id` = `posts`.`thread_id`

And

SELECT `threads`.`id` AS `threads.id` , `posts`.`id` , `posts`.`content`
FROM `threads` , `posts`
WHERE `threads`.`id` = `posts`.`thread_id`

They both return same data.

Answer 1

WHERE clause filtering result set which is returned by JOIN, so this is a difference.

As long as you are using INNER JOIN there is no differences neither performance nor execution plan, in case of any OUTER JOIN query would produce different execution plan.

Also pay attention to what is said in the MySql online doc:

Generally, you should use the ON clause for conditions that specify
how to join tables, and the WHERE clause to restrict which rows you
want in the result set.