[Koha] stuck and i need a mysql clue

[Erik Lewis]

How would I exclude NULL values from the author, title, or isbn fields  
in the query.

SELECT b.biblionumber, b.author, b.title, SUBSTR(i.isbn,1,10)
FROM biblio b, biblioitems i
WHERE b.biblionumber = i.biblionumber;


I know it should be easy, but I'm not having much luck.