Default Column for the References Clause

If the referenced table is different from the referencing table, you do not need to specify the referenced column; the default column is the primary-key column (or columns) of the referenced table. If the referenced table is the same as the referencing table, you must specify the referenced column.

The following example creates a new column in the cust_calls table, ref_order. The ref_order column is a foreign key that references the order_num column in the orders table. 
ALTER TABLE cust_calls 
   ADD ref_order INTEGER 
   REFERENCES orders (order_num) 
   BEFORE user_id;

When you place a referential constraint on a column or set of columns, and a duplicate or unique index already exists on that column or set of columns, the index is shared.