1. These constraints arise frequently. Every relation arising from a relationship set has referential integrity constraints.

• Figure 6.1 shows an n-ary relationship set R relating entity sets E1,E2, .. .En.

              Figure 6.1: An n-ary relationship set

• Let Ki denote the primary key of Ei.
• The attributes of the relation scheme for relationship set R include K1∪ K2 ∪. . .∪Kn.
• Each Ki in the scheme for R is a foreign key that leads to a referential integrity constraint.

2. Relation schemes for weak entity sets must include the primary key of the strong entity set on which are existence dependent. This is a foreign key, which leads to another referential integrity constraint.