a column of a table can be declared to have a built-in datatype called a REF to allow it to “point to”a row object of a object table.
example:
create type customer_t as object(…);
create type agent_t as object(…);
create type product_t as object(…);
create type order _t as object
(
ordno int,
month char(3),
cid char(4),
aid char(3),
pid char(3),
qty int,
dollars double precision,
ordcust ref customer_t,
ordagent ref agent_t,
ordprod ref product_t
);