Discussion
Consider the following ER diagram.
Minimum number of tables needed to represent M, N, P, R1, R2 is
Answer: B
The minimum number of tables is 3.
- R1 is many-to-one from M to P, and M takes part totally, so R1 can be merged into M's table by adding P1 as a foreign key: M(M1, M2, M3, P1).
- P needs its own table: P(P1, P2).
- N is a weak entity identified through R2, so the identifying relationship is merged into N's table, whose key is P1 plus the partial key N1: N(P1, N1, N2).
That is three tables. Five would give R1 and R2 tables of their own, which the cardinalities make unnecessary, and two would lose either P or the weak entity N.
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