Introduced are the concepts of closed entity-relationship diagrams
(CERDs), and of partially, and generalized commutative diagrams (which are
particular types of CERDs). Briefly presented is the elementary mathematical
data model (EMDM), used to enable more accurate database scheme design.
Provided is an algorithm for assisting database designers in modeling CERDs.
Shown is that every such CERD should be thoroughly scrutinized according to
this algorithm, even if some of them might prove ”uninteresting” in the end.