Plugin Activation

<?php
 
function mpm_activate(){
	        add_filter('posts_join', 'mpm_activatefunc');
		
		remove_filter('posts_join', 'mpm_activatefunc');
}
 
function mpm_activatefunc(){
 
$pageposts = $wpdb->get_results("SELECT * FROM $wpdb->posts, $wpdb->postmeta 
        WHERE $wpdb->posts.ID = $wpdb->postmeta.post_id", OBJECT);
        
      if ($pageposts): foreach ($pageposts as $post): $post_id = $post->ID;; 
          $isproduct = get_post_meta($post_id, 'isproduct', $single = true);  //is the "isproduct" meta-key set
          $priceset = get_post_meta($post_id, 'price', $single = true); //or a price set
          $pliset = get_post_meta($post_id, 'pli', $single = true);
    
    if ((isset($priceset)) && (!empty($priceset))) {
       delete_post_meta($post_id, 'isproduct');
       add_post_meta($post_id, 'isproduct', '1');
    }
    elseif ((isset($pliset)) && (!empty($pliset))) {
        delete_post_meta($post_id, 'isproduct');
       add_post_meta($post_id, 'isproduct', '1');
       }
    else {
      delete_post_meta($post_id, 'isproduct');
       add_post_meta($post_id, 'isproduct', '0');
    }
 
 
endforeach;
endif;
 }
 
 register_activation_hook( __FILE__, 'mpm_activate' );