TRUE, FALSE, NULL, Empty

None, True, False are replaced in SQL with NULL, TRUE, FALSE. All of the following work:

DQ("Book", "id, name").where("in_print is True")
DQ("Book", "id, name").where("in_print is not True")
DQ("Book", "id, name").where("in_print is False")
DQ("Book", "id, name").where("in_print == True")

To test for NULL, use None:

DQ("Book", "id, name").where("name is not None")

If you want to test for an empty or non-empty string, use LENGTH():

DQ("Book", "id, name").where("length(name) > 0")