mysql headache nightmare! (multiple meta_keys)

Hi folks,

A bit of background info:
I’m building a theme for a band and this theme is going to include tour posters as featured images within the custom post type of ‘concerts’.
I already have the concert listings working as I’d like, using a custom field ‘_event_date’ to determine the order in which they’re displayed and whether they’re displayed at all (no concerts prior to today’s date are displayed).

This was all well and good until I tried to have it check for the existence of ‘_thumbnail_id’ as a meta_key also and now within the query the meta_key for _thumbnail_id is cancelling out the meta_key for _event_date.

Not good.

Here’s the code I’m using just now to get the upcoming concerts out of the database:

global $wpdb;
$blog_prefix = $wpdb->prefix;
$today = strtotime("today");

$upcoming_concert_posters_query = "
	SELECT *
	FROM ".$blog_prefix."posts, ".$blog_prefix."postmeta
	WHERE ID = post_id
	AND post_type = 'concerts'
	AND meta_key = '_event_date'
	AND meta_value >= ".$today."
	AND post_status = 'publish'
	ORDER BY meta_value ASC
	LIMIT 100
";

$upcoming_concert_posters = $wpdb->get_results($upcoming_concert_posters_query, OBJECT);

How can I change this to retain this current functionality but add an additional conditional which checks for the existence of _thumbnail_id?

As you might have guessed, I suck with MySQL queries and any help would be greatly appreciated ??

Cheers,
Rab