What is Entity Relationship Diagram? Webopedia Definition
Attributes are not shown on the Entity-Relationship Model but are recorded in the underlying data dictionary which contains the definitions of. This definition explains the meaning of entity relationship diagram, also known data model, illustrating specific attributes and relationships among data points. Entities, defined as tables that hold specific information (data); Relationships, defined . In the entity relationship diagram, shown in Figure , each attribute is.
A simple attribute, such as grade, is one component that is atomic. If we consider the name in two parts, last name and first name, then the name attribute is a composite.
A composite attribute, such as "Emanuel Vagas", has multiple components, such as "Emanuel" and "Vagas"; and each component is atomic or composite.
We illustrate this composite nature in the ER model by branching off the component attributes, such as in Figure 4.
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- ER Model - Basic Concepts
Definition A simple attribute is one component that is atomic. A composite attribute has multiple components, each of which is atomic or composite. ER diagram notation for composite attribute domain, name Another way to classify attributes is either as single-valued or multi-valued. For an entity an attribute, such as StudentGrade, usually holds exactly one value, such as 93, and thus is a single-valued attribute.
However, two lab assistants might assist in a laboratory section. 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.
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.Entities, Attributes, and Relationships
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. For example, we can have two binary relationship types associating the student and team types, TeamMemberOf and LeaderOf. In the former case, a student entity is a member of a team entity; in the later case, a student can be a leader of a team. We illustrate the situation in Figure Finally, the serial number on a car engine is an example of a one-to-one relationship; each engine has just one serial number, and each serial number belongs to just one engine.
We often use the shorthand terms 1: N for one-to-one, one-to-many, and many-to-many relationships, respectively. The number of entities on either side of a relationship the cardinality of the relationship define the key constraints of the relationship.
There are many relationships that may at first seem to be one-to-one, but turn out to be more complex. For example, people sometimes change their names; in some applications, such as police databases, this is of particular interest, and so it may be necessary to model a many-to-many relationship between a person entity and a name entity. Redesigning a database can be time-consuming if you assume a relationship is simpler than it really is.
In an ER diagram, we represent a relationship set with a named diamond.
entity-relationship diagram (model)
The cardinality of the relationship is often indicated alongside the relationship diamond; this is the style we use in this book. The ER diagram representation of the customer and product entities, and the sale relationship between them.
Partial and Total Participation Relationships between entities can be optional or compulsory. In our example, we could decide that a person is considered to be a customer only if they have bought a product.
On the other hand, we could say that a customer is a person whom we know about and whom we hope might buy something—that is, we can have people listed as customers in our database who never buy a product.
These are referred to as the participation constraints of the relationship. In an ER diagram, we indicate total participation with a double line between the entity box and the relationship diamond.
ER Model Basic Concepts
From time to time, we encounter cases where we wonder whether an item should be an attribute or an entity on its own. For example, an email address could be modeled as an entity in its own right. When in doubt, consider these rules of thumb: Is the item of direct interest to the database?
Objects of direct interest should be entities, and information that describes them should be stored in attributes. Our inventory and sales database is really interested in customers, and not their email addresses, so the email address would be best modeled as an attribute of the customer entity. Does the item have components of its own? If so, we must find a way of representing these components; a separate entity might be the best solution. In the student grades example at the start of the chapter, we stored the course name, year, and semester for each course that a student takes.
Can the object have multiple instances? If so, we must find a way to store data on each instance. The cleanest way to do this is to represent the object as a separate entity. In our sales example, we must ask whether customers are allowed to have more than one email address; if they are, we should model the email address as a separate entity.
Is the object often nonexistent or unknown? If so, it is effectively an attribute of only some of the entities, and it would be better to model it as a separate entity rather than as an attribute that is often empty. Consider a simple example: The ER diagram representation of student grades as a separate entity Entity or Relationship? An easy way to decide whether an object should be an entity or a relationship is to map nouns in the requirements to entities, and to map the verbs to relations.
All else being equal, try to keep the design simple, and avoid introducing trivial entities where possible; i. Intermediate Entities It is often possible to conceptually simplify many-to-many relationships by replacing the many-to-many relationship with a new intermediate entity sometimes called an associate entity and connecting the original entities through a many-to-one and a one-to-many relationship.
A passenger participates in an M: Any given flight can have many passengers with a booking. Any given passenger can have bookings on many flights. Hence, we can consider the many-to-many relationship to be in fact two one-to-many relationships, one each way.
This points us to the existence of a hidden intermediate entity, the booking, between the flight and the passenger entities. The requirement could be better worded as: The intermediate booking entity between the passenger and flight entities Each passenger can be involved in multiple bookings, but each booking belongs to a single passenger, so the cardinality of this relationship is 1: Similarly, there can be many bookings for a given flight, but each booking is for a single flight, so this relationship also has cardinality 1: Since each booking must be associated with a particular passenger and flight, the booking entity participates totally in the relationships with these entities.
We described partial and total participation earlier in Partial and Total Participation. For example, we generally call family members by only their first name or nickname. Where ambiguity exists, we add further information such as the surname to clarify our intent.