6.18

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

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

b. If the two $X$ s 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 $B$

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a. Rename the attribute $X$ to something that describes more info in both entity sets, so that it can have a different name in both the entity sets.

b.

Case 1: $X$ is the primary key for $A$ but not $B$

In this case we would create a foreign-key constraint on the attribute $X$ of the entity set $B$ referencing the entity set $A$.

Case 2: $X$ is the primary key for both $A$ and $B$

In this case we merge the two entity sets $A$ and $B$ into one entity set $C$. The new entity set $C$ will have the attribute $X$ as a primary key and moreover it will have all the other attributes of $A$ and $B$ (making proper attribute name modifications, if needed).

Case 3: $X$ is not the primary key for $A$ nor for $B$

In this case we can create a new entity set called $C$ having a single attribute $X$. And the attribute $X$ in $A$ and $B$ will be a foreign-key referencing the attribute $X$ in the entity set $C$. This also gives room for storing more information about the attribute $X$ in side of the entity set $C$.