How to query for posts with either one or another custom field

Question

Your example is a bit hard to understand, as it seems to query for meta_values of X,Y,Z,AB,ID,IA. I hope you can still make sense of the below posted code:

// query posts that have on of the custom values for the custom meta key 'my_key' 
// and/or one of the custom meta values for the meta key 'your_key'
$posts = get_posts( array(
     'post_type' => 'my_post_type'
    ,'meta_query' => array(
         array(
             'key' => 'my_key'
            ,'value' => array( 'AB', 'ID', 'IA' )
            ,'compare' => 'IN'
         )
        ,array(
             'key' => 'your_key'
            ,'value' => array( 'AB', 'ID', 'IA' )
            ,'compare' => 'IN'
     )
) );

Other comparison values would be 'LIKE', 'NOT LIKE', 'NOT IN', 'BETWEEN', 'NOT BETWEEN' where the later twos are reserved for numeric comparison and need an extra meta key/value pair: 'type' => 'numeric'.