Maintaining integrity between 3 tables when reference could be through or direct Say we have the following 3 tables:

tUsers (ID, name, countryID, regionID)
------
1  |  Joe  |  1   |  1
2  |  Sue  | NULL |  2
3  |  Bob  |  2   |  NULL
4  |  Amy  | NULL |  NULL

tCountries (ID, name, regionID)
------
1  |  USA   |  1
2  |  China |  3

tRegions (ID, name)
------
1  |  North America
2  |  Central America
3  |  East Asia
4  |  Europe


Is there a way from the DB to prevent a user from having mismatched Country/Region while still allowing one or both to be NULL?  For example, the query UPDATE tUsers SET regionID=4 WHERE ID = 3 would fail since Bob's country is China which is not in region Europe.
3 foreign keys version

create table tRegions (
  ID int not null primary key, 
  name  varchar(20)
); 

create table tCountries (
  ID int not null primary key, 
  name varchar(20), 
  regionID int, 
  --any superset of PK is undoubtly UNIQUE
  constraint cu1 unique (ID, regionID),
  foreign key (regionID) references tRegions(ID)
  );

create table tUsers (
 ID int not null primary key, 
 name varchar(20), 
 countryID int ,
 regionID int,
 foreign key (regionID) references tRegions(ID),
 foreign key (countryID) references tCountries(ID),
 foreign key (countryID,regionID) references tCountries(ID,regionID )
)

insert tRegions (ID, name)
values
(1,'North America'),
(2,'Central America'),
(3,'East Asia'),
(4,'Europe') 

insert tCountries(ID, name, regionId)
values
(1,'USA',1),
(2,'China',3)

insert tUsers (ID, name, countryID, regionId)
values
(1,'Joe', 1  ,1),
(2,'Sue',NULL,2),
(3,'Bob', 2  ,NULL),
(4,'Amy',NULL,NULL)

go
--OK
update tUsers set RegionId=3
where ID=3
select * from tUsers
go
--fails
update tUsers set CountryId=2
where ID=2
select * from tUsers