Page:
{embed="cart/display_user_carts" member_id="<?php global $SESS; echo $SESS->userdata['member_id'];?>"}

Sub-template:
{exp:query sql="SELECT id, member_id, member_username, cart_name FROM cart_requests WHERE member_id = {embed:member_id} ORDER BY timestamp DESC"}
<p>{cart_name}</p>
{/exp:query}

http://www.thenestedfloat.com/articles/expressionengines-sql-query-module

Expression Engine Dynamic Query

Smarty