Check Constraints Or Triggers

Hi, I´m facing teh following situation:

This are just sample table names, but should do for discussing
purpouses.

Create table Invoice
(
InvoiceID Integer Not Null,
CustomerType Integer Not Null,
CustomerCode Integer Not Null,
Amount DECIMAL(10,2) Not Null,
.................
)

Create Table Type1Customer
(
CustomerCode Integer Not Null,
...............................
)


Create Table Type2Customer
(
CustomerCode Integer Not Null,
...............................
)

I need to add a way to restrict the CustomerType and CustomerCode,
in the Invoice table to the correct values.
This means that if customerType equals 1 the customerCode should be
checked against Type1Customer and if customerType equals 2 the
customerCode should be checked against Type2Customer.

I succesfully created a check constraint. That ensures that the valid
values exists when the rows in the Invoice table are inserted or
updated, but doesn´t prevent from deleting records from tables
Type1Customer and Type2Customer that are referenced from the Invoice
table.

Are triggers the only way to go?

Thanks in advance

Sebastián streiger