Entity Relationship Diagram Symbols

About Entity-relationship Diagram

The so-called semantic modeling method nowadays is commonly used in database structure design. Semantic modeling is modeling data structures, based on the meaning of these data. Different variants of the entity-relationship diagrams are used as a tool for the semantic modeling. ER-model based diagrams have three main components: an entity, a relation and attributes. An entity is a class of similar objects, information about which should be taken into account in the model. Each entity must have a name, expressed by a noun in the singular. Examples of entities can be such classes of objects as "Supplier", "Employee", "Invoice". Each entity in the model is depicted in the form of a rectangle with the name.

While determining the type of entity it is necessary to be sure that each instance of an entity may be indistinguishable from any other instance of the same entity.

Relation is presented in the form of non-directional line connecting two entities or leading from the entity to itself. In the place of "docking" an entity uses:

• three-point entry to the entity rectangle, if many instances of the entity may be used for this relation
• single-point entry, if relation can (or should) participate in only one instance of the entity.

Relation between entities "ticket" and "passenger" connects passengers and tickets. One side of the relation with a label "for" is a many-to-many relation which allows to relate more than one ticket to a passenger, and each ticket must be linked to any passenger. Another side of the relation with a label "have" is a zero or one relation, which shows that each ticket can belong to only one passenger, and the passenger is not obliged to have at least one ticket. Attribute of an entity is a detail or feature that is used for verification, identification, classification, numerical characteristics or expressing status of the entity. Attribute names are recorded in the rectangle representing an entity under entity name. For instance, an entity "human being" may have such attributes as gender, year of birth, first name, etc.

It is the specialized Entity Relationship diagram symbols, and the meanings of those symbols

Entity Relationship Diagram Symbols — Chen notation

Symbol	Shape Name	Symbol Description
Entities
	Entity	An entity is represented by a rectangle which contains the entity’s name.
	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.
	Associative Entity	An entity used in a many-to-many relationship (represents an extra table). All relationships for the associative entity should be many
Attributes
	Attribute	In the Chen notation, each attribute is represented by an oval containing atributte’s name
	Key attribute	An attribute that uniquely identifies a particular entity. The name of a key attribute is underscored.
	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.
	Derived 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.
Relationships
	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
	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.

Entity Relationship Diagram Symbols — Crow’s Foot notation

Symbol	Meaning
Relationships (Cardinality and Modality)
	Zero or One
	One or More
	One and only One
	Zero or More
Many - to - One
	a one through many notation on one side of a relationship and a one and only one on the other
	a zero through many notation on one side of a relationship and a one and only one on the other
	a one through many notation on one side of a relationship and a zero or one notation on the other
	a zero through many notation on one side of a relationship and a zero or one notation on the other
Many - to - Many
	a zero through many on both sides of a relationship
	a zero through many on one side and a one through many on the other
	a one through many on both sides of a relationship
	a one and only one notation on one side of a relationship and a zero or one on the other
	a one and only one notation on both sides

There are many notation styles that express cardinality:

• Information Engineering Style;
• Chen Style;
• Bachman Style;
• Martin Style;

Example 1. Entity relationship diagram symbols, Chen's and Crow’s Foot notation

Entity-Relationship model making possibility to describe a database by abstract way. The data in database tables can be the point to data in other tables - for instance, your entry in the database could point to several entries.

Example 2. Entity Relationship Diagram (ERD) Software (Windows)

ConceptDraw DIAGRAM software thanks to the included Entity-Relationship Diagram (ERD) solution gives the ability to describe visually a database using the Chen's and Crow’s Foot notation icons for drawing ER diagrams (ERD).

Example 3. Entity Relationship Diagram (ERD) Software (macOS)

The Entity-Relationship Diagram (ERD) solution from ConceptDraw Solution Park extends ConceptDraw DIAGRAM ector graphics and diagramming software with the ability to describe a database using the Entity-Relationship model. It provides entity relationship diagram examples, templates and library of vector stencils with ERD symbols.

Use ConceptDraw DIAGRAM enhanced with Entity-Relationship Diagram (ERD) solution to design you own entity relationship diagrams using Chen's or Crow's Foot notations.