Understanding Weak Entities in Databases

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Question:

What is the definition of weak entity in database?

Answer:

In database design, a weak entity is an entity that cannot be uniquely identified by its own attributes alone. Instead, it relies on a "strong" or "owner" entity to provide part of its identification. Weak entities are typically used in situations where the existence of the weak entity is dependent on the strong entity.

Key Characteristics of Weak Entities:

  1. Partial Key: A weak entity has a partial key, which is an attribute or a set of attributes that can uniquely identify the weak entity only when combined with the primary key of the strong entity.

  2. Existence Dependency: A weak entity cannot exist without its associated strong entity. If the strong entity is deleted, the weak entity is also deleted.

  3. Identifying Relationship: The relationship between a weak entity and its strong entity is called an identifying relationship. This relationship is typically represented with a double line in an Entity-Relationship (ER) diagram.

  4. No Primary Key: Weak entities do not have a primary key of their own; instead, they use a combination of their partial key and the primary key of the strong entity to form a composite key.

Example:

Consider a scenario with two entities: Order (strong entity) and OrderItem (weak entity).

  • Order has a primary key, such as OrderID.
  • OrderItem might have a partial key, such as ItemNumber, but it cannot be uniquely identified without the OrderID. Therefore, the combination of OrderID and ItemNumber serves as the composite key for OrderItem.

In this case, OrderItem is a weak entity because it depends on Order for its identification and existence.