-- create a table
CREATE TABLE supplier
(supplier_id numeric(10),
supplier_name varchar2(20),
contact_name varchar2(20),
CONSTRAINT pk_supplier PRIMARY KEY (supplier_id)
);
-- insert some values
insert into supplier values (111,'Ram', 'abc');
insert into supplier values (222,'Hari', 'def');
-- fetch some values
select * from supplier;

CREATE TABLE products
(product_id numeric(10),
supplier_id numeric(10),
CONSTRAINT fk_supplier FOREIGN KEY (supplier_id) REFERENCES supplier(supplier_id)
);

insert into products values(777,111);
insert into products values(777,11); ----error
insert into products values(777,111); ---duplicate allowed
insert into products values(777,NULL);----NULL allowed
insert into products values(NULL,111);
Select * from products;