Wordpress create table on Plugin activation if not exists

<?php global $wpdb; $table1 = $wpdb->prefix . "my_table"; if($wpdb->get_var( "show tables like '$table1'" ) != $table1) { $sql = "CREATE TABLE `". $table1 . "` ( "; $sql .= " `post_id` BIGINT NULL, "; $sql .= " `field1` VARCHAR(250) NULL, "; $sql .= " `field2` VARCHAR(250) NULL, "; $sql .= " INDEX `index_by_post_id` (`post_id`) "; $sql .= ") ENGINE=MyISAM DEFAULT CHARSET=latin1 ; "; require_once( ABSPATH . '/wp-admin/includes/upgrade.php' ); dbDelta($sql); }