Consider a weak entity set in DBMS. Cardinality ratio 'strong entity set: weak entity set' in their binary identifying relationship is :

Question

Accepted Answer

The correct option is (2):
The cardinality ratio for the binary identifying relationship between a strong entity set (owner) and a weak entity set (dependent) must always be One-to-Many (1:N), specifically from the strong entity to the weak entity. This relationship is fundamental to the definition of a weak entity, which is an entity that cannot be uniquely identified by its own attributes alone and is existence-dependent on the strong entity. The "One" side indicates that each instance of the weak entity set must be associated with exactly one instance of the strong entity set, as it must inherit the strong entity's primary key to complete its own composite primary key. The "Many" side indicates that a single instance of the strong entity can own, or be associated with, multiple instances of the weak entity (e.g., one employee having many dependents). This total participation on the weak entity's side, combined with the 1:N ratio, ensures every weak entity instance is fully and unambiguously identified by its owner.

The incorrect option is (1):
A One-to-One (1:1) relationship is too restrictive for an identifying relationship and is not a requirement for weak entities. While a weak entity could technically be modeled with a 1:1 relationship, this is not the general rule. A 1:1 cardinality implies that each strong entity instance can have at most one weak entity instance. The inherent utility of a weak entity lies in its ability to model multiple, existence-dependent details for a single owner, such as multiple line items on an invoice or multiple dependents for an employee. If the relationship were truly forced to be 1:1, the unique partial key would often be sufficient to identify the weak entity, or the attributes of the weak entity could simply be merged into the strong entity, making the distinct weak entity set redundant.

The incorrect option is (3):
The cardinality ratio Many-to-One (N:1) from the strong to the weak entity set is fundamentally incorrect. This ratio implies that one instance of the weak entity could be associated with multiple instances of the strong entity. This violates the critical rule that a weak entity must be uniquely and totally identified by the primary key of a single, unique identifying owner strong entity. If it were dependent on multiple owners, its composite key would be ambiguous and non-unique, which is a structural impossibility in relational database design. The dependence is always on *one* strong entity for its unique identity.

The incorrect option is (4):
A Many-to-Many (M:N) relationship is structurally incompatible with the definition of an identifying relationship for a weak entity. In an M:N relationship, a single instance of the weak entity set would be linked to many instances of the strong entity set, and vice versa. As established, a weak entity must derive its complete identity from *one* instance of the strong entity. Furthermore, implementing an M:N relationship requires an intermediate *associative* entity, which is a strong entity itself, and thus this relationship type cannot represent the direct, existence-dependent link that defines a weak entity.