# Weak Entities, Specialisation and Aggregation — Database Fundamentals

Source: https://www.skillbyai.com/en/database-fundamentals/e-advanced

> Model weak entities, IS-A hierarchies and aggregation.

## Beyond the basics

A **weak entity set** cannot be identified by its own attributes alone; it depends on an **owner** (identifying) entity through an **identifying relationship**. A *dependent* of an employee is identified by the employee's ID plus the dependent's name (the **partial key** or discriminator, drawn with a dashed underline). Weak entities are drawn with a double rectangle and the identifying relationship with a double diamond, and they always have total participation. **Specialisation** divides an entity set into sub-classes (an EMPLOYEE is a PROFESSOR or STAFF), and **generalisation** is the reverse, combining similar entities into a super-class; both form **IS-A** hierarchies where sub-classes inherit attributes. Constraints describe them: **disjoint** versus **overlapping** (can an entity belong to several sub-classes?) and **total** versus **partial** (must every super-class entity belong to some sub-class?). **Aggregation** treats a relationship as an entity so it can take part in another relationship, for example a *monitors* relationship between a manager and a *works-on* relationship.

## Weak entities and an IS-A hierarchy

Textual notation for the extended ER concepts.

```text
weak entity DEPENDENT (name [partial key], relation, dob)
  identifying relationship HAS_DEPENDENT: EMPLOYEE 1 --- N DEPENDENT  (total for DEPENDENT)
  full key of DEPENDENT = (emp_id, name)

EMPLOYEE (emp_id, name, salary)
   |  ISA  (disjoint, total)
   +-- PROFESSOR (research_area)
   +-- STAFF     (shift)

aggregation: [EMPLOYEE --WORKS_ON-- PROJECT] --MONITORED_BY-- MANAGER
```

## Rooms in a building

Room 101 means nothing on its own; it becomes unique only as "Room 101 in Block C". A room is a weak entity whose partial key (101) is completed by its owner (the block).

**Quiz:** What identifies a weak entity?

- [ ] Its own primary key alone
- [x] Its partial key combined with the key of its owner entity
- [ ] A random number
- [ ] The relationship's attributes only

*Answer:* Its partial key combined with the key of its owner entity. A weak entity is identified by the owner's key plus its own discriminator.