Question

Consider the representation of the ternary relationship (Figure (a) using three binary relationships (Figure (b)) (1) Construct a simple instance of E, A, B, C, RA, RB, Rc, that cannot correspond to any instance of A, B, C, and R. (11) Modify the E-R diagram (b) to introduce constraints that will guarantee that any instance of E, A, B, C, RA, RB, Rc, that satisfies the constraints will correspond to an instance of A, B, C, and R.

Answer 1

i).

Let,E={e₁,e₂},A={a₁,a₂},B={b₁},C={c1},R_A={(e₁,a₁),(e₂,a₂)},R_B={(e₁,b₁)} and R_C={(e₁,c₁)}.We see that because of the tuple   (e₂,a₂),no instance of R exists which corresponds to E,R_A,R_B,R_C.

ER Diagram:  

ii).The idea is to introduce total participation constraints between E and the relationships R_A,R_B,R_C so that every tuple in E has a relationship with A,B and C.

ER Diagram:

Thus,this is the solution.