title: mysql finding not exist records in mutiple tables
body: <pre><code>    CREATE TABLE `Students` (
    `id` int(11) NOT NULL,
    `name` varchar(500) NOT NULL,
  ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
  
  INSERT INTO `Students` (`id`, `name`) VALUES
  (1, 'Student 1'), 
  (2, 'Student 2'),
  (3, 'Student 3');
  
  
  
  CREATE TABLE `lessons` (
    `id` int(11) NOT NULL,
    `name` varchar(500) NOT NULL,
  ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
  
  INSERT INTO `lessons` (`id`, `name`) VALUES
  (1, 'Lesson 1'),
  (2, 'Lesson 2'),
  (3, 'Lesson 3');
  
  
  
  CREATE TABLE `completed` (
    `student` int(11) NOT NULL,
    `lesson` varchar(500) NOT NULL,
    `completed` int(11) NOT NULL,
  ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
  
  
  
  INSERT INTO `completed` (`student`, `lesson`, `completed`) VALUES
  (1, 2, 1),
  (3, 3, 1),
  (2, 1, 1);
  </code></pre>
  
  <p>we are adding students who completed lessons to "completed" table. we need to get 5 student with lesson which id not exists in "completed" table.</p>
  
  <p>example output is; </p>
  
  <pre><code>1, 1
  1, 3
  2, 2
  2, 3
  3, 1
  3, 2
  </code></pre>
  
  <p>thank you</p>