Identifying vs Non-Identifying

In an ER Diagram, a relationship between entities can either be:

1. Identifying Relationship → when the child entity’s existence and primary key depend on the parent entity.
2. Non-Identifying Relationship → when the child entity exists independently and just refers to the parent entity via a foreign key.

Identifying Relationship

• Occurs when a weak entity depends on a strong entity.
• The child entity does not have a primary key of its own.
• The parent entity’s primary key becomes part of the child’s composite primary key.
• In ERD (Chen notation): double diamond.
• In Crow’s Foot ERD: the child’s PK includes the FK (solid line).

Example: Bank Account & Transaction

• Transaction is a weak entity because it cannot exist without an Account.
• Each transaction is identified by (Account_ID + Transaction_ID).

ER Representation:

[ACCOUNT] ◉──(Identifies)──◉ [TRANSACTION]

ACCOUNT
-------------------
Account_ID (PK)
Account_Type
Balance

TRANSACTION
-------------------
Account_ID (PK, FK)
Transaction_ID (PK)
Amount
Date

• Transaction is identified only when combined with Account_ID.
• This makes the relationship identifying.

Non-Identifying Relationship

• Occurs when the child entity is a strong entity with its own primary key.
• The parent’s primary key is added as a foreign key only (not part of child’s PK).
• In ERD: single diamond.
• In Crow’s Foot ERD: the child’s PK does not include the FK (dashed line or regular line).

Example: Teacher & Course

• A Course exists independently of a teacher.
• Each course has its own course_id as a primary key.
• But we add teacher_id as a foreign key to show who teaches it.

ER Representation:

[TEACHER] ──(Teaches)── [COURSE]

TEACHER
-------------------
teacher_id (PK)
Name
Department

COURSE
-------------------
course_id (PK)
course_name
Credits
teacher_id (FK)

• Course has its own independent key (course_id).
• teacher_id is just a foreign key reference.
• This makes the relationship non-identifying.

Key Differences between Identifying and Non-Identifying Relationships

Feature	Identifying Relationship	Non-Identifying Relationship
Entity Type	Connects strong → weak entity	Connects strong → strong entity
Child’s Primary Key	Includes parent’s PK	Independent of parent’s PK
Existence	Child cannot exist without parent	Child can exist without parent
ERD Symbol	Double diamond (Chen) / solid line (Crow’s Foot)	Single diamond (Chen) / dashed/regular line (Crow’s Foot)
Example	Account–Transaction	Teacher–Course

Real-Life Analogy

• Identifying → Like a transaction receipt: it only makes sense when tied to a specific account.
• Non-Identifying → Like a course: it exists on its own, but you can assign a teacher to it.