Last modified: November 6, 2025

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Weak Entity Sets

• A weak entity can't be uniquely identified by its own attributes alone.
• It depends on a strong entity (another table/entity) for its primary key.
• From the example: University(name) → Strong entity (can be identified just by name). Team(sport, number, universityName) → Weak entity (needs universityName to uniquely identify a Team).

Without the university's name, you might have multiple teams with the same sport and number — but which university they belong to would be unclear.

Why do we care about Weak Entities?

1. Avoid Duplicating Keys:

   • Instead of copying the whole key into every weak entity, you reference it simply.

2. Reflect Logical Structure:

   • It models real-world dependency — e.g., a Team exists only because a University exists.

3. Automatic Deletion:

   • If the strong entity (like a University) is deleted, the dependent weak entities (like its Teams) should automatically be deleted too.

Implementation

CREATE TABLE Company (
    CompanyID NVARCHAR(10) PRIMARY KEY,
    CompanyName NVARCHAR(50),
    Address NVARCHAR(50)
);

CREATE TABLE Product (
    CompanyID NVARCHAR(10),
    ProductID NVARCHAR(10),
    ProductName NVARCHAR(50),
    Categiry NVARCHAR(50),
    PRIMARY KEY (CompanyID, ProductCode),
    FOREIGN KEY (CompanyID) REFERENCES Company(CompanyID) ON DELETE CASCADE,
);

Weak Relationship

A relationship is weak (identifying) only if it connects a weak entity to its owner (strong entity) and is used to define its primary key.