What Is a Weak Entity in ER Diagrams? Explained with Simple Example

🧠 What Is a Weak Entity?
🔍 Key Characteristics of a Weak Entity
📘 Example: Course and Course Section
🔗 How to Represent a Weak Entity in an ER Diagram
🧠 Summary
✅ Final Thoughts

🧠 What Is a Weak Entity?

In an Entity-Relationship (ER) model, a weak entity is an entity that cannot be uniquely identified by its own attributes. Unlike regular (strong) entities, it depends on another entity—called the strong entity—to form a complete unique identifier.

In short:

A weak entity can’t exist on its own and must be tied to another entity to make sense.

🔍 Key Characteristics of a Weak Entity

Here are the key features that define a weak entity:

Feature	Description
Depends on a Strong Entity	It needs another entity to identify it uniquely.
Partial Key	It has a special attribute (or set) called a partial key that, combined with the strong entity’s key, makes a unique ID.
Existence Dependency	It cannot exist without the associated strong entity.
ER Diagram Representation	Represented with a double rectangle, and its identifying relationship is shown with a double diamond.

📘 Example: Course and Course Section

Let’s take a university database example:

✅ Strong Entity: `Course`

Attributes: CourseID, CourseName

✅ Weak Entity: `CourseSection`

Attributes: SectionNumber, Instructor, Time
Partial Key: SectionNumber

Relationship: `Has Section` (between `Course` and `CourseSection`)

🧠 Why is CourseSection weak?

The section number alone (like “Section 1”) doesn't mean much.
But if we say “Math 101, Section 1”, it becomes clear.
So, we need both CourseID and SectionNumber to uniquely identify a course section.

🔗 How to Represent a Weak Entity in an ER Diagram

In ER diagrams:

Use a double rectangle for the weak entity
Connect it to the strong entity using a double diamond for the identifying relationship
Draw the partial key (e.g., SectionNumber) underlined with a dotted line

🧠 Summary

Term	Meaning
Weak Entity	An entity that can’t be uniquely identified on its own
Strong Entity	The entity that gives identity to the weak entity
Partial Key	Attribute(s) that help identify the weak entity when combined with the strong entity's key
ER Diagram Symbol	Double rectangle for weak entity, double diamond for identifying relationship

✅ Final Thoughts

Weak entities are essential in modeling real-world relationships where a record doesn’t make sense unless tied to another. By understanding how to use partial keys and identifying relationships, you can build accurate, normalized data models.

📥 Next Steps: Try creating your own ER diagram with weak entities using examples like:

Bank and its Account Transactions
Building and its Apartments