can anyone explain what is this sql query?

[paragasu <paragasu gmail com>]

[snip]
#define CONTENTS_DEL_SQL "DELETE FROM order_contents WHERE order_id =
##oid::gint AND product_ref = ##-0::gchararray AND quantity =
##-2::gint AND discount = ##-3::gdouble"
[/snip]

i take this from the official gnome-db documentation
http://library.gnome.org/devel/libgnomedb-3.0/unstable/ch04s03.html

unfortunately, i don't know what the ##oid:gint or ##-0::gchararray or
##-2::gint or ##-3::gdouble mean (inside the delete query). And i cannot find
any documentation on it. i want to know what the -0, -2 or -3 mean ..

can anyone point me  a link for me to read more, i am more than happy if anyone
can explain it to me.. thank you