Question

Consider the ER model:
E1 (A11, A12, A13)
E2 (A21, A22, A23)
Where:
- A21 is the key of E2
- A22 is a multivalued attribute
- R12 is a many-to-many relationship between E1 and E2
Find the minimum number of relations required to convert this ER model into tables.

Hint:

In ER to Relational mapping:
Each strong entity → 1 relation
Each multivalued attribute → 1 separate relation
Each many-to-many relationship → 1 separate relation

Answer 1

Correct Answer: 4

Step 1: Understanding the Topic
This question is about converting an Entity-Relationship (ER) model into a relational schema (a set of tables). There are standard rules for this conversion process, and we need to count the number of tables (relations) required based on the components described.
Step 2: Key Approach - Applying Standard Conversion Rules
We will systematically apply the mapping rules for each component of the ER model:

Each strong entity set maps to its own table.
Each many-to-many (M:N) relationship set maps to its own table.
Each multivalued attribute requires its own separate table.
Step 3: Detailed Explanation
Let's count the number of tables needed based on the rules:

Table for Entity E1: E1 is a strong entity set, so it requires one table. \[ \text{Table 1: E1(A11, A12, A13)} \]
Table for Entity E2: E2 is also a strong entity set, so it requires another table. Its multivalued attribute, A22, will not be included here. \[ \text{Table 2: E2(A21, A23)} \]
Table for Multivalued Attribute A22: The multivalued attribute A22 of entity E2 needs a separate table. This table will include the key of E2 (which is A21) to link back to the parent entity. \[{Table 3: E2_A22(A21, A22)}\]
Table for M:N Relationship R12: The many-to-many relationship R12 between E1 and E2 requires its own table. This table's primary key will be a composite key formed from the primary keys of the participating entities (assuming A11 is the key for E1 and A21 is the key for E2). \[ \text{Table 4: R12(A11, A21)} \]
Summing these up, we need a total of 1 + 1 + 1 + 1 = 4 tables.
Step 4: Final Answer
The minimum number of relations (tables) required is 4. \[ \boxed{4} \]