Amazing! Let's summarize what we've learned.

We can:

• Insert data using NULLs for unknown values in a row:

  INSERT INTO student VALUES
  (21, 'Tom', NULL, 'Muller');
  

• Change values using NULL and UPDATE:

  UPDATE exam 
  SET written_exam_score = NULL
  WHERE id = 5;
  

• Change a value in a column or delete a row if the column contains a NULL:

  UPDATE student
  SET last_name = 'unknown' 
  WHERE last_name IS NULL;

  DELETE FROM student 
  WHERE last_name IS NULL;

• Update or delete using AND and OR to meet certain conditions:

  DELETE FROM exam 
  WHERE subject = 'Spanish' 
    AND (written_exam_score IS NULL 
      OR oral_exam_score IS NULL)