Question

I am currently using the following two queries:

$get_forum=$this->prepare("SELECT * FROM ".TBL_PREFIX."forums WHERE id=:forum_id");
$get_forum->bindParam(":forum_id", $forum_id);
$get_forum->execute();

$forum=$get_forum->fetchAll();

$get_parent=$this->prepare("SELECT * FROM ".TBL_PREFIX."forums WHERE id=:parent_id");
$get_parent->bindParam(":parent_id", $forum["parent_id"]);
$get_parent->execute();

I would like to combine them into one query something like this where it uses the bound param or a value from a field in the database. I don’t know what the parent ID is until I get the forum data back from the database.

Parent forums have a parent_id of -1, while child forums have the parent_id set to the id of the parent.

$get_forums=$this->prepare("SELECT * FROM ".TBL_PREFIX."forums WHERE id=:forum_id OR forum_id=parent_id");
$get_forums->bindParam(":forum_id", $forum_id);
$get_forums->execute();

Is it possible to use a field value while inside the query itself to compare it with another value?

Database sample:
http://sqlfiddle.com/#!2/2b354/1

Answer 1

You can make a self-join:

SELECT *
FROM   forums parent JOIN forums child ON parent.forum_id = child.parent_id
WHERE  parent.id = ? OR child.id = ?