Database: book_1
Question: Give the titles of books authored by Plato that have a sale price lower than the average sale price across all books.
SQL:
SELECT T1.title FROM Book AS T1 JOIN Author_book AS T2 ON T1.isbn  =  T2.isbn JOIN Author AS T3 ON T2.Author  =  T3.idAuthor WHERE T3.name  =  "Plato" AND T1.saleprice  <  (SELECT avg(saleprice) FROM Book)