Question
What do you understand by the concept of weak entity used in data modelling? Explain the relevance of owner entity type, partial key and identifying relationship in the context of such modelling.
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Answer
The weak entities refer to those entities which do not have their own key attributes or identities. The weak entities are identified on the basis of their relation with the specific entities from another entity type in combination with some of their attribute values. These other entity types are regarded as identifying or owner entity type. The entity type, whose entities are in relationship with the weak entities are referred to as identifying or owner entity type. This can be described in other words, as the entities which are identified because of their relationship with their parent entity or dominant entity is referred to as weak entities or child entity type (or subordinate entity type), while the parent entity is referred to as identifying entity type.
Accordingly, the relationship through which the weak entity type relates to its owner entity is known as identifying relationship of weak entity. For a unique identification of the weak entity that is related to the same owner entity, a set of attributes is used which is known as partial key. Sometimes, the partial key is also known as the discriminator. This is because it is used to uniquely identify the weak entities that are related to the same parent entity. For example, if we assume that no two children of the same employee have the same first name, then the attribute Name of Children is the partial key that can be used to identify the children (weak entities) that are related to the same employee (owner entity).
Accordingly, the relationship through which the weak entity type relates to its owner entity is known as identifying relationship of weak entity. For a unique identification of the weak entity that is related to the same owner entity, a set of attributes is used which is known as partial key. Sometimes, the partial key is also known as the discriminator. This is because it is used to uniquely identify the weak entities that are related to the same parent entity. For example, if we assume that no two children of the same employee have the same first name, then the attribute Name of Children is the partial key that can be used to identify the children (weak entities) that are related to the same employee (owner entity).
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