6.10

Consider a many-to-one relationship $R$ between entity sets $A$ and $B$. Suppose the relation created from $R$ is combined with the relation created from $A$. In SQL, attributes participating in a foreign key constraint can be null. Explain how a constraint on total participation of $A$ in $R$ can be enforced using NOT NULL constriant in SQL.

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The foreign-key attribute in $R$ corresponding to the primary key of $B$ should be made NOT NULL. This ensures that no tuple of $A$ which is not related to any entry in $B$ under $R$ can come in $R$. For example, say a is a tuple in $A$ which has no corresponding entry in $R$. This means when $R$ is combined with $A$, it would have a foreign-key attribute corresponding to $B$ as NULL, which is not allowed.