In the Entity Relationship (ER) Model, a Weak Entity is an entity type that cannot be uniquely identified by its own attributes alone. It depends on a strong entity (also called owner entity) for its identification. Weak entities are used to represent real‑world situations where an object only makes sense in the context of another object.

Weak entities are important for modeling dependent data such as bank transactions, order line items, or dependents of an employee.

What is a Weak Entity?

A Weak Entity has:

• No primary key of its own; it cannot be uniquely identified by its own attributes.

• A partial key (also called discriminator): a set of attributes that, together with the identifier of the strong entity, make the weak entity unique.

• An identifying (or identifying relationship) relationship with a strong entity.

The whole key of a weak entity is formed by combining the primary key of the strong entity and the partial key of the weak entity.

Example: Weak Entity in College Database

Consider entities:

• STUDENT (strong entity)

  • Key: Roll_No

• ENROLLMENT (weak entity)

  • Attributes: Semester, Grade

  • Partial key: Semester

Here, an ENROLLMENT record is only meaningful for a particular STUDENT and in a specific SEMESTER. Two different students can have the same Semester value, but STUDENT_Roll_No + Semester together uniquely identify an enrollment.

So the key of ENROLLMENT is:

• Composite key = {Roll_No, Semester}

  • Roll_No comes from the strong entity STUDENT.

  • Semester is the partial key of the weak entity ENROLLMENT.

Symbols in ER Diagram

In ER diagrams, a weak entity is usually shown as a double‑bordered rectangle, while the strong entity is a single‑bordered rectangle. The relationship between them is called an identifying relationship and is often shown with a double‑bordered diamond or special notation.

Example sketch (textual):

• [STUDENT] —— (identifies) ——> [ENROLLMENT]

• STUDENT has single border.

• ENROLLMENT has double border (weak entity).

Why Use Weak Entities?

• To model dependent data that only exists in the context of another object.

• To avoid creating meaningless unique IDs where the natural key is formed by a strong entity key plus a partial key.

• To clearly show in the design that certain entities cannot exist independently.

Mapping Weak Entity to Relational Model

When converting a weak entity to a relational schema:

• The weak entity becomes a table.

• The primary key of the weak entity table is the composite key formed by:

  • The primary key of the strong entity (copied as a foreign key).

  • The partial key attributes of the weak entity.

• All other attributes of the weak entity become regular columns.

Example SQL‑like structure:

sql
CREATE TABLE ENROLLMENT (
    Roll_No   INT,            -- from STUDENT (foreign key)
    Semester  VARCHAR(10),    -- partial key
    Grade     CHAR(2),
    PRIMARY KEY (Roll_No, Semester),
    FOREIGN KEY (Roll_No) REFERENCES STUDENT(Roll_No)
);

Summary

A Weak Entity in DBMS is an entity type that cannot be uniquely identified without the help of a strong entity. It has a partial key and participates in an identifying relationship with the strong entity. The composite key for the weak entity includes the strong entity’s primary key and the partial key. Weak entities are useful for modeling dependent data such as enrollment records, transactions, or line items, and they map naturally to relational tables with composite primary keys.