$6\mathrm{.}18$ Consider two entity sets A and B that both have the attribute X $($among others whose names are not relevant to this question$).$

If the two X s are completely unrelated, how should the design be improved?

If the two Xs represent the same property and it is one that applies both to A and to B$,$ how should the design be improved? Consider three subcases:

X is the primary key for A but not B

X is the primary key for both A and B

X is not the primary key for A nor for B