What is Entity Relationship Diagram (ERD)?
An entity relationship model, also called an entity-relationship (ER) diagram, is a graphical It's sketched using the diamond shape. It's one or more attributes that uniquely identify a weak entity for a given owner entity. We don't define one column for name (if it's composite), instead we split it up into. An entity–relationship model (ER model for short) describes interrelated things of interest in a Consequently, the ER model becomes an abstract data model, that defines a data or information structure which can and relationships may be called entity-attribute-relationship diagrams, rather than entity–relationship models. What is an ER diagram (ERD)?; When to draw ER Diagrams? a rounded rectangle, with its name on top and its attributes listed in the body of the entity shape.
What is Entity Relationship Diagram (ERD)?
In other words, there must not be two or more records that share the same value for the primary key attribute. The ERD example below shows an entity 'Product' with a primary key attribute 'ID', and a preview of table records in database. Foreign Key Also known as FK, a foreign key is a reference to a primary key in table. It is used to identify the relationships between entities. Note that foreign keys need not to be unique.
Multiple records can share the same values.
The ER Diagram example below shows an entity with some columns, among which a foreign key is used in referencing another entity. Relationship A relationship between two entities signifies that the two entities are associated with each other somehow. For example, student might enroll into a course. The entity Student is therefore related with Course, and the relationships is presented as a connector connecting between them. Cardinality Cardinality defines the possible number of occurrence in one entity which are associated to the number of occurrences in another.
When present in an ERD, the entities Team and Player are inter-connected with a one-to-many relationship. In an ER diagram, cardinality is represented as a crow's foot at the connector's ends. The three common cardinal relationships are one-to-one, one-to-many, and many-to-many.
One-to-One cardinality example A one-to-one relationship is mostly used to split an entity in two to provide information concisely and make it more understandable.
The figure below shows an example of one-to-one relationship. One-to-Many cardinality example A one-to-many relationship refers to the relationship between two entities X and Y in which an instance of X may be linked to many instances of Y, but an instance of Y is linked to only one instance of X. The figure below shows an example of one-to-many relationship.
Many-to-Many cardinality example A many-to-many relationship refers to the relationship between two entities X and Y in which X may be linked to many instances of Y and vice versa. The figure below shows an example of many-to-many relationship. Consequently, the LabAssistant attribute for the entity LabSection is multi-valued. A multi-valued attribute has more than one value for a particular entity.
We illustrate this situation with a double oval around the lab assistant type, LabAssistant see Figure 5.
- Entity–relationship model
Definition For a particular entity, an entity attribute that holds exactly one value is a single-valued attribute. ER diagram notation for multi-valued attribute domain, LabAssistant A derived attribute can be obtained from other attributes or related entities.
For example, the radius of a sphere can be determined from the circumference. We request the derived attribute with a dotted oval and line, such as in Figure 6. ER diagram notation for derived attribute, radius An attribute or set of attributes that uniquely identifies a particular entity is a key.
However, to determine the class we need a composite key that consists of several attributes, such as catalogue number, section, semester, and year. In the ER diagram of Figure 7we underline the composite key, class.
The figure shows another attribute DragExpWeek of LabSection that stores the week of the semester in which the drag experiment occurs. Definition An attribute or set of attributes that uniquely identifies a particular entity is a key. A composite key is a key that is a composite of several attributes. For example, the student entity type is related to the team entity type because each student is a member of a team.
In this case, a relationship or relationship instance is an ordered pair of a specific student and the student's particular physics team, such as Emanuel Vagas, PhysFA04where PhysFA04 is Emanuel's team number. Figure 8 illustrates three relationships. Unfortunately, Itnatios Trekas had to drop the course and retake it another semester.
Consequently, his name is associated with two team numbers. We arrange the diagram so that the relationship reads from left to right, "a student is a member of a team. Definition A relationship type is a set of associations among entity types. A relationship or relationship instance is an ordered pair consisting of particular related entities.
The degree of a relationship type is the number of entity types that participate. Thus, the LabSecMemberOf relationship type of Figure 9 has degree 2, which we call a binary relationship type.
To clarify the role that an entity plays in each relationship instance, we can label a connecting edge with a role name that indicates the purpose of an entity in a relationship. It is an intensional model. At the latest since Carnapit is well known that: The first part comprises the embedding of a concept in the world of concepts as a whole, i. The second part establishes the referential meaning of the concept, i. Extension model[ edit ] An extensional model is one that maps to the elements of a particular methodology or technology, and is thus a "platform specific model".
The UML specification explicitly states that associations in class models are extensional and this is in fact self-evident by considering the extensive array of additional "adornments" provided by the specification over and above those provided by any of the prior candidate "semantic modelling languages". It incorporates some of the important semantic information about the real world.
Plato himself associates knowledge with the apprehension of unchanging Forms The forms, according to Socrates, are roughly speaking archetypes or abstract representations of the many types of things, and properties and their relationships to one another.
Limitations[ edit ] ER assume information content that can readily be represented in a relational database. They describe only a relational structure for this information. They are inadequate for systems in which the information cannot readily be represented in relational form[ citation needed ], such as with semi-structured data.
For many systems, possible changes to information contained are nontrivial and important enough to warrant explicit specification. An alternative is to model change separately, using a process modeling technique.
Additional techniques can be used for other aspects of systems. For instance, ER models roughly correspond to just 1 of the 14 different modeling techniques offered by UML.