The use of including upgrade.php when building custom queries

I started building a plugin in which I need to build some custom queries with wpdb object. It’s the first time I use it. So I checked out how to use it. I watched some examples and there is something which is messing up my mind : upgrade.php

Actually when I create a custom query, this one only works when I include upgrade.php this way :

require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
$custom_posts = $wpdb->get_results( 'SELECT * FROM ' . self::TABLE . '', OBJECT_K );
$wpdb->show_errors();
echo $wpdb->last_query;
return $custom_posts;

This is just one of the method’s example I can privide.

What is the use of upgrade.php ? I mean, if I don’t put it, the query doesn’t work. Could someone explain, please, cause there’s nothing in the codex concerning that.

Am I really oblidged to use it ? why ? How ?

Thanks

UPDATE : Here is the complete code.

/**
 * On activate or uninstall plugin
 */
register_activation_hook( __FILE__, array( 'Cpt', 'init' ) );
register_deactivation_hook( __FILE__, array( 'Cpt', 'uninit' ) );
register_uninstall_hook( __FILE__, array( 'Cpt', 'uninit' ) );

global $wpdb;

class Cpt
{

    CONST TABLE = 'cpt';

    /**
     * Constructor
     */
    public function init()
    {
        /**
         * Creation table for cpt
         */
        $sql = "CREATE TABLE IF NOT EXISTS `" . self::TABLE . "` (
          id mediumint(9) NOT NULL AUTO_INCREMENT,
          time datetime DEFAULT '0000-00-00 00:00:00' NOT NULL,
          name tinytext NOT NULL,
          UNIQUE KEY id (id)
        );";
        require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
        $wpdb->query($sql);

        self::convert_to('post', 'custom');
    }

    public function uninit() {
        /**
         * Delete table for cpt
         */
        require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
        //$wpdb->query("DROP TABLE IF EXISTS `cpt`;");
        //$this->delete_cpt("Custom post 1");
    }

    /**
     * Add a custom post type
     */
    public function add_cpt($name) {
        /**
         * Add a single data
         */
        $time = (string)date('dmY');
        require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
        $wpdb->insert( 
            self::TABLE, 
            array(
                'time'=>$time, 
                'name'=>$name
            ) 
        );
        /*$wpdb->show_errors();
        echo $wpdb->last_query;
        die();*/
    }

    /**
     * Delete a custom post type
     */
    public function delete_cpt($name) {
        /**
         * delete a single data
         */
        require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
        $wpdb->delete( 
            self::TABLE, 
            array( 
                'name'=>$name 
            ) 
        );
        $wpdb->show_errors();
        echo $wpdb->last_query;
        die();
    }

    /**
     * Get all custom post types
     */
    public static function get_ctps() {
        /**
         * delete a single data
         */
        require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
        $custom_posts = $wpdb->get_results( 'SELECT * FROM ' . self::TABLE . '', OBJECT_K );
        $wpdb->show_errors();
        echo $wpdb->last_query;
        return $custom_posts;
    }

    /**
     * Get a single custom post type
     */
    public function get_ctp($name) {
        /**
         * get a single data
         */
        require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
        $custom_post = $wpdb->get_results( 'SELECT name FROM ' . self::TABLE . ' WHERE name = $name', OBJECT_K );
        $wpdb->show_errors();
        echo $wpdb->last_query;
        print_r($custom_post);
        //return $custom_post;
    }

    /**
     * Post type conversion
     */
    public static function convert_to($from, $to) {

        // Get the posts of the type $from
        require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
        $wpdb->update( 'wp_posts', array('post_type'=>$to), array('post_type'=>$from) );
        $wpdb->show_errors();
        echo $wpdb->last_query;
    }

    /**
     * Generate admin menu with all custom post types
     */
    public function generate_menu() {

        $custom_posts = self::get_ctps(); 
    }
}

No, and in fact you should not include the wp-admin/includes/upgrade.php file in a plugin. There is no real valid use case for doing so. It doesn’t do anything for you or add any useful functions for a plugin.

As for your question, you say “it doesn’t work” but you fail to define what that means. Providing the error messages you receive is useful when you’re asking questions.

I do note that in a number of functions, you’re trying to call the functionality in $wpdb without declaring global $wpdb; first. Try replacing the require_once with that instead.