Adds filters to a query

functions.php

<?php 
// don't use the php tags when copying this in your own theme's functions.php

function add_new_query_vars($public_query_vars) {
  $public_query_vars[] = 'posts_fields';
  $public_query_vars[] = 'posts_join';
  $public_query_vars[] = 'posts_where';
	$public_query_vars[] = 'posts_orderby';
	return $public_query_vars;
}

function new_posts_fields ($fields) {
	// Make sure there is a leading comma
	$new_fields = get_query_var('posts_fields');
	if ($new_fields) $fields .= (preg_match('/^(\s+)?,/', $new_fields)) ? $new_fields : ", $new_fields";
	return $fields;
}

function new_posts_join ($join) {
	$new_join = get_query_var('posts_join');
	if ($new_join) $join .= ' ' . $new_join;
	return $join;
}

function new_posts_where ($where) {
	$new_where = get_query_var('posts_where');
	if ($new_where) $where .= ' ' . $new_where;
	return $where;
}

function new_posts_orderby ($orderby) {
	$new_orderby = get_query_var('posts_orderby');
	if ($new_orderby) $orderby = $new_orderby;
	return $orderby;
}

add_filter('query_vars', 'add_new_query_vars');
add_filter('posts_fields','new_posts_fields');
add_filter('posts_join','new_posts_join');
add_filter('posts_where','new_posts_where');
add_filter('posts_orderby','new_posts_orderby');

/*  
now you can query like this on your theme template files:

global $wpdb;
$args = array(
  'posts_per_page' => 5,
  'posts_where' => " AND $wpdb->posts.post_title LIKE 'A%'", // query for post titles that start with the letter a
);
query_posts($args);

*/