CREATE TABLE CoBought (
   Book1    INTEGER,
   Book2    INTEGER,
   Copurchased#   INTEGER,
   PRIMARY KEY (Book1, Book2),
   FOREIGN KEY (Book1) REFERENCES Book (Isbn)
   FOREIGN KEY (Book2) REFERENCES Book (Isbn)
   CHECK (Book1 <> Book2)
   CHECK (CoPurchased# >= 0)
   CHECK (NOT EXISTS (SELECT *   
                      FROM CoBought CB  
                      WHERE CB.Book1 = Book2  AND Book1 = CB.Book2))  
              
)
    /* instead of the CHECK (NOT EXISTS …) in CoBought above, 
       we might instead have changed CHECK (Book1 <> Book2) to 
       CHECK (Book1 < Book2), thus forcing an ordered pair and
       disallowing the entry of a “duplicate” (unordered) pair. */