Get only used meta_values

I am not sure if I get your question right, and thus don’t know if this answer goes into the right direction… Is this what you’re trying to achive? 

$apart_locations = $wpdb->get_col( "SELECT meta_value FROM $wpdb->postmeta AS wppm
    LEFT JOIN $wpdb->posts AS wpp ON wppm.post_id = wpp.ID
    WHERE wppm.meta_key = 'postal_location' AND wpp.post_status="publish"
    ORDER BY wppm.meta_value ASC" );

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