ALTER TABLE t_invoice 
ADD customer_id integer 
CONSTRAINT FK_customer_id FOREIGN KEY (customer_id) 
REFERENCES t_customer (customer_id)

create table a(id int primary key)
create table b(id1 int)

ALTER TABLE b 
ADD idtest integer 
CONSTRAINT FK_test_id FOREIGN KEY (id1) 
REFERENCES a (id)

select * from a
select * from b


drop table b
drop table a