MySQL - How to create a trigger to prevent creating another table after 20 book reviews

I need help creating a trigger for this SQL problem:

  • BookAuthor(book, year, author, earnings)
  • BookReference(book, year, referencedBook, referencedYear, times)
  • BookReview(book, year, reviewer, score)
  • BookPublish(book, year, publisher, price, num)

No reviewer may review more than 20 books published in the same year