In relational database design, merging a relationship table $R$ into an entity table $A$ means adding the primary key of $B$ (as a foreign key) directly into table $A$.
1. Cardinality Constraint
For the table for $R$ to be merged with $A$, each entity in $A$ must be associated with at most one entity in $B$.
If an entity in $A$ could relate to multiple entities in $B$ (a one-to-many relationship from $A$ to $B$), merging them would violate the first normal form ($1\text{NF}$) because table $A$ would need to store multiple values of $B$'s primary key for a single row of $A$.
Therefore, the relationship $R$ from $A$ to $B$ must be many-to-one. This ensures that for every row in $A$, there is exactly one corresponding value from $B$ to place in that row's foreign key column.
2. Participation Constraint
Now we consider whether $A$'s participation in $R$ should be total or partial.
Total Participation: Every entity in $A$ must be associated with an entity in $B$. If we merge $R$ into $A$, every row in table $A$ will have a valid, non-null foreign key value pointing to $B$. This avoids any NULL values in the foreign key column.
Partial Participation: Some entities in $A$ do not participate in $R$. If we merge the tables, the rows in $A$ that don't participate in $R$ will have NULL values in the foreign key column. While this is technically possible in standard SQL, it is not considered a clean or ideal relational design due to NULL insertion.
To merge the table without introducing NULL values or structural issues, total participation is required.
Correct Answer: C. Relationship $R$ is many-to-one and the participation of $A$ in $R$ is total
