Strong and Weak Entities

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An identifying attribute is unique, singular, and required.

• correspond one-to-one to, or identify, entity instances

Strong Entities

A strong entity has one or more identifying attributes.

• when implemented as a table,
  • one of the identifying attributes may become the primary key

Weak Entities

A weak entity does not have an identifying attribute.

• usually has a relationship, called an identifying relationship, to another entity, called an identifying entity
  • identifying entity must be singular and required in an identifying relationship
• in ER diagram,
  • identifying relationship has a diamond next to the identifying entity
    • diamond replaces the entity’s cardinality symbols
      • because it is always singular and required
• identifying relationships replace identifying attributes

Identifying Entities

• A weak entity is identified by:
  • a strong entity
  • another weak entity
  • or by several entities
• identifying attribute may be complex when weak entity is identified by a weak entity or multiple entities
  • identifying attribute depends on business rules and may not be apparent in the ER diagram

Database Design

• During conceptual design, the database designer distinguishes strong and weak entities
  • For each weak entity, the identifying relationship is noted
  • Weak entities and identifying relationships are documented in the glossary and ER diagram
• Software tools usually allow users to choose from alternative conventions and automatically switch between conventions

Distinguish Strong and Weak Entities

Step	Activity
3A	Identify strong and weak entities.
3B	Determine the identifying relationship(s) for each weak entity.
3C	Document weak entities and identifying relationships in glossary and ER diagram.