Mysql join for thread and its replies

Answer 1

Don't use a join in this case as it does not give any advantage and instead it will slow down the queries. Rather do two separate queries:

mysql_query('SELECT *
         FROM threads AS t
         WHERE t.t_id = '.mysql_escape_strings($_GET['t_id']));

And:

mysql_query('SELECT *
         FROM replies AS r
         WHERE r.t_id = '.mysql_escape_strings($_GET['t_id']));

This will be much faster.