Figure 4 depicts the referential constraints that exist among the various tables in the university database. The boxes represent tables. The unidirectional-directional arrow implies a referential constraint from the parent table to the referencing table. For example, in constraint number 16 a foreign key exists in the Class table that references a primary key in its parent table, Course.
Note: Figure 4 also serves as a dependency graph. It tells you what tables must be populated before others when doing the physical design.