21. The medium of claim 20 , wherein the field of the first dataset is a primary key and the field of the second dataset is a foreign key that references the primary key.
21. The medium of claim 20 , wherein the field of the first dataset is a primary key and the field of the second dataset is a foreign key that references the primary key. 22. The medium of claim 21 , wherein the constraint specification includes a representation of a foreign key to primary key relationship between the field of the second dataset and the field of the first dataset.