ER Model

The Entity-Relationship (ER) model is a conceptual data model used to describe the structure of a database at a high level, independent of any specific database system. It was introduced by Peter Chen in 1976.

Core Concepts

Entities

An entity is a real-world object or concept that is distinguishable from other objects. Entities of the same kind are grouped into entity types (e.g., Customer, Product, Order). A single occurrence is called an entity instance.

Attributes

Attributes describe the properties of an entity type.

Type	Description	Example
Simple	Atomic, indivisible value	FirstName, Age
Composite	Made up of sub-attributes	Address = (Street, City, ZIP)
Multivalued	Can hold multiple values	PhoneNumbers
Derived	Computed from another attribute	Age derived from BirthDate

The key attribute (primary key) uniquely identifies each entity instance, e.g., CustomerID.

Relationships

A relationship describes an association between two or more entity types. Like entities, relationships are grouped into relationship types (e.g., a Customer places an Order). Relationships can also have their own attributes (e.g., a WorksFor relationship might carry a StartDate).

Cardinality

Cardinality defines how many instances of one entity can be associated with instances of another.

Type	Description	Example
1:1	One instance relates to exactly one other	One person has one passport
1:N	One instance relates to many others	One customer places many orders
N:M	Many instances relate to many others	Students enroll in multiple courses; courses have multiple students

Participation further specifies whether every entity instance must take part in a relationship:

• Total participation (mandatory): Every instance must be in at least one relationship, e.g., every order must belong to a customer.
• Partial participation (optional): Some instances may not participate, e.g., not every customer has placed an order.

Weak Entities

A weak entity cannot be uniquely identified by its own attributes alone. It depends on a strong (owner) entity for its identity.

• The weak entity has a partial key (discriminator) that is unique only within the context of its owner.
• The relationship connecting a weak entity to its owner is called an identifying relationship.

Example: OrderItem is a weak entity. Its partial key LineNumber is only unique within a specific Order. The full identity is (OrderID, LineNumber).

Example: Order Management

The following example shows a Customer who places Orders, each consisting of one or more OrderItems. OrderItem is a weak entity, since a LineNumber only makes sense in the context of a specific order.

┌──────────────────┐                      ┌──────────────────┐
│    Customer      │                      │     Order        │
├──────────────────┤                      ├──────────────────┤
│ CustomerID (PK)  │─── 1 ── places ── N ─│ OrderID (PK)     │
│ Name             │                      │ OrderDate        │
│ Email            │                      └────────┬─────────┘
└──────────────────┘                               │ 1
                                                   │
                                               contains
                                            (identifying)
                                                   │
                                                   │ 1..*
                                          ╔════════╧═════════╗
                                          ║    OrderItem     ║
                                          ╠══════════════════╣
                                          ║ ~LineNumber      ║
                                          ║  Quantity        ║
                                          ╚══════════════════╝

Mapping to Relational Tables

ER concept	Relational mapping
Entity type	Table
Attribute	Column
Key attribute	Primary key
1:N relationship	Foreign key on the N-side table
N:M relationship	Junction table with foreign keys to both entity tables
Weak entity	Table with composite primary key (owner key + partial key)