ERD Notations in Data Modeling. Part 2

Chen Notation

Patrycja Dybka, Community Manager at Vertabelo

by
Patrycja Dybka
Community Manager at Vertabelo

Posted: August 2, 2014 | Updated: December 9, 2015

Continuing our trip through different ERD notations, let’s review the Chen ERD notation.

Peter Chen, who developed entity-relationship modeling and published his work in 1976, was one of the pioneers of using the entity relationship concepts in software and information system modeling and design. The Chen ERD notation is still used and is considered to present a more detailed way of representing entities and relationships.

Entities

An entity is represented by a rectangle which contains the entity’s name.

In the Chen notation, we distinguish types of entities:

  1. entity

    Entity in the Chen ERD notation

  2. weak entity – an entity that cannot be uniquely identified by its attributes alone. The existence of a weak entity is dependent upon another entity called the owner entity. The weak entity’s identifier is a combination of the identifier of the owner entity and the partial key of the weak entity.

    Weak entity in the Chen ERD notation

  3. associative entity – an entity used in a many-to-many relationship (represents an extra table). All relationships for the associative entity should be many

    Associative entity in the Chen ERD notation

Attributes

In the Chen notation, each attribute is represented by an oval containing atributte’s name:

Attribute in the Chen ERD notation

Attributes are linked to the entity with straight lines:

Chen ERD notation: entity with some atributes

There are some specific types of attributes:

  1. key attribute – an attribute that uniquely identifies a particular entity. The name of a key attribute is underscored:

    Attributes in Chen ERD notation: key attribute

    For example, since the Vehicle Identification Number (VIN) is a unique code used to identify individual vehicles (no two vehicles have the same VIN), “VIN” can be considered as the key attribute for the “CAR” entity:

    Chen ERD notation: entity with the key attribute

  2. partial key attribute (discriminator) – an attribute that, when combined with the key attribute of the owner entity, provides a unique identification for the weak entity. We underline the discriminator with a dashed line:

    Attributes in Chen ERD notation: partial key attribute

  3. multivalued attribute – an attribute that can have many values (there are many distinct values entered for it in the same column of the table). Multivalued attribute is depicted by a dual oval:

    Attributes in Chen ERD notation: multivalue attribute

    For example, each user can have several different hobbies, therefore the “hobby” can be considered as a multivalued attribute for the “user” entity:

    Chen ERD notation: entity with a multivalued attribute

  4. derived attribute (or computed attribute) – an attribute whose value is calculated (derived) from other attributes. The derived attribute may or may not be physically stored in the database.

    In the Chen notation, this attribute is represented by dashed oval:

    Attributes in Chen ERD notation: derived attribute

    For example, having given the price excluding VAT and the VAT rate, we can calculate the price including VAT:

    Chen ERD notation: entity with a derived attribute


Some attributes can be further subdivided into smaller parts. For example, the attribute “address” can be subdivided into street name, street number, apartment number, city, state, zip code, and country.

These are called composite attributes and are depicted as follows:

Chen ERD notation: entity with a composite attribute


Relationships

In the Chen notation, a relationship is represented by a diamond (rhombus) containing the relationship’s name.

Two kinds of relationship are distinguished:

  1. strong relationship – a relationship where entity is existence-independent of other entities, and PK of Child doesn’t contain PK component of Parent Entity. A strong relationship is represented by a single rhombus:

    Strong relationship's symbol in the Chen ERD notation

  2. weak (identifying) relationship – a relationship where Child entity is existence-dependent on parent, and PK of Child Entity contains PK component of Parent Entity. This relationship is represented by a double rhombus:

    Weak relationship's symbol in the Chen ERD notation


Optionality of a relationship

  1. Similarly to the Barker’s notation, a mandatory relationship is represented by a solid line:

    Chen ERD notation - mandatory relationship line

  2. An optional relationship is represented by a dashed line like in Barker’s notation:

    Chen ERD notation - optional relationship line


Cardinality

The degree of relationship (cardinality) is represented by characters “1”, “N” or “M” usually placed at the ends of the relationships:

  • one-to-one (1:1)

    The employee can manage only one department, and each department can be managed by one employee only:

    Chen ERD notation - One-to-one relationship

  • one-to-many (1:N)

    The customer may place many orders, but each order can be placed by one customer only:

    Chen ERD notation - One-to-many relationship

  • many-to-one (N:1)

    Many employees may belong to one department, but one particular employee can belong to one department only:

    Chen ERD notation - Many-to-one relationship

  • many-to-many (M:N)

    One student may belong to more than one student organizations, and one organization can admit more than one student:

    Chen ERD notation - Many-to-many relationship


Participation constraints

An entity set may participate in a relation either totally or partially.

  • Total participation means that every entity in the set is involved in the relationship, e.g., each student must be guided by a professor (there are no students who are not guided by any professor). In the Chen notation, this kind of relation is depicted as a double line.

  • Partial participation means that not all entities in the set are involved in the relationship, e.g., not every professor guides a student (there are professors who don’t). In the Chen notation, a partial participation is represented by a single line.

The relationship shown above means that each student, without exception, must be guided by one chosen professor, and one – but not every – professor can guide many students. So there is no student that is not guided by a professor, and on the other hand there can be professors who don't guide any students.


Now, let’s see how our example with book and chapter (presented in the previous part) looks like in the Chen notation.

Book-and-chapter schema presented in the Chen notation


 
 

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