Er Model Practice Questions

S1 is TRUE: A 1:1 relationship with total participation on both sides can be merged into one table. S2 is TRUE: A weak entity cannot exist without its owner, so it always has total participation in the identifying relationship. S3 is TRUE: Ternary relationships can have M:N:P cardinalities. S4 is FALSE: Partial participation means the entity set does NOT necessarily participate in the relationship - i.e., some entities may not be associated. Existential dependency applies to total participation or weak entities, not partial participation.

For a 1:1 relationship, all of A, B, C, D are valid representations: A: Valid - FK of A placed in B's table. B: Valid - FK of B placed in A's table. C: Valid - A separate table can always be created (though redundant for 1:1, it is correct). D: Valid - If both participate totally in a 1:1 relationship, merging is lossless and preferred. E: Invalid - Placing both PKs into only one entity table would cause data redundancy and anomalies.

Option C is INCORRECT. In a 1:N relationship, the foreign key goes the other way - the primary key of the entity on the 1-side is placed as a foreign key in the table of the entity on the N-side (not the other way around). This avoids NULL values and redundancy. All other options A, B, and D correctly describe standard ER-to-relational mapping rules.

In an M:N relationship, every entity of E1 can be associated with every entity of E2. Maximum relationship instances = |E1| × |E2| = 5 × 4 = 20. This is the Cartesian product and represents the maximum possible tuples in the relationship table.

A weak entity does not have its own primary key. Its identity is determined by combining its discriminator (partial key) - here Item_No - with the primary key of its owner entity - here Cust_ID. Therefore, the primary key of the Order table in the relational schema is the composite key (Cust_ID, Item_No). Item_No alone may not be unique across all customers, but the combination is unique.

Cardinality defines relationship mapping (1:1, 1:N, M:N).

Entity tables: Doctor (1), Patient (2), Ward (3) - 3 tables. R1 (Doctor-Patient M:N): requires a separate table → 1 table. R2 (Patient-Ward M:1 with total participation of Patient): Patient already has total participation, so Ward's primary key can be added as FK in Patient table → no new table needed. R3 (Doctor-Ward M:N): requires a separate table → 1 table. Total = 3 (entities) + 1 (R1) + 1 (R3) = 5 tables.

A: FALSE - In 1:N (A side is '1'), each entity in A can be associated with MANY entities in B. B: TRUE - Each entity in B maps to at most one entity in A (B is on the 'N' side). C: TRUE - In a 1:N relationship, the primary key of the '1' side (A) is added as a foreign key in the 'N' side table (B), avoiding a separate table. D: FALSE - A separate table is mandatory only for M:N relationships; 1:N can be handled via foreign key. E: TRUE - Total participation of B means every entity in B must participate in R, and since B is on the N-side (each B maps to one A), every B must be associated with exactly one A.

Since every employee MUST work in exactly one department, Employee has total participation (double line in ER diagram). Since a department MAY have zero employees, Department has partial participation (single line). Each employee works in exactly one department (many employees to one department), so cardinality is M:1 from Employee to Department.