How can I turn these multiple mysql_querys into 1 query?

Question

I have two tables: users and threads. When a thread is created, it will store the user_id in the table as author_id. I want to display the thread name and the username of its author with the same query. I am currently using two querys as shown:

$query2 = mysql_query("SELECT author_id FROM threads WHERE id = $threadId") or die(mysql_error());
$result2 = mysql_fetch_array($query2);
$author_id = $result2['author_id'];
$query3 = mysql_query("SELECT username FROM users WHERE id = $author_id") or die(mysql_error());
$result3 = mysql_fetch_array($query3);
$author_name = $result3['username'];

Answer 1

<?php
$sql = '
    SELECT t.name, u.username
    FROM threads t
    JOIN users u ON t.author_id = u.id
    WHERE t.id = ' . (int)$threadId . '
';
list($thread_name, $author_name) = mysql_fetch_row(mysql_query($sql));

P.S. Mysql php extension is deprecated.

Answer 2

Try this query:

SELECT username 
FROM users 
WHERE id = (SELECT author_id 
            FROM threads 
            WHERE id = $threadId 
            LIMIT 1)

Note: Limit 1 is not mandatory as id is unique.