2008/12/4 Gavin King <gavin@hibernate.org>:
  

Sometimes you need null values, sometimes you don't. Java should let
you declare whether a value accepts null as part of the type
declaration (i.e. @Nullable Foo foo), and non-nullable should be the
default. But Java doesn't have this facility, so we just have to live
with it.
    

A good practice should be to use "boolean" for not nullables (the primitive one)
as it can't be null; reserve the wrapper Boolean for values which can be null.
This has nothing to do with SQL of course ;-)