The ER model defines the conceptual view of a database. It works around real-world entities and the associations among them. At view level, the ER model is considered a good option for designing databases.
Creation of an ER diagram, which is one of the first steps in designing a database, helps the designer s to understand and to specify the desired components of the database and the relationships among those components.
An ER model is a diagram containing entities or "items", relationships among them, and attributes of the Entity relationship model and the relationships.
Definition The Entity-Relationship ER modela high-level data model that is useful in developing a conceptual design for a database.
To make the description of the model more complete, we consider the example of a physics department at a college that maintains a database Entity relationship model experimental results. Throughout a laboratory, students collaborate and share their results and access data sets from other semesters on a computer system.
For example, in the laboratory session on "Freely Falling Objects with Significant Drag," students determine the drag coefficient by dropping dust balls from different heights and measuring the times they take to fall.
Each team enters its results into the distributed database, and the class analyzes the data. After a team enters data into the web-accessed database, all students can obtain the measurements simultaneously.
To simplify the analysis, we assume that the database only stores results related to this experiment over a period of several years.
Entity An entity is a real-world item or concept that exists on its own.
In our example, a particular student such as, "Emanuel Vagas"team, lab section, or experiment is an entity. The set of all possible values for an entity, such as all possible students, is the entity type.
In an ER model, we diagram an entity type as a rectangle containing the type name, such as student see Figure 2. Definition An entity is a real-world item or concept that exists on its own. The set of all possible values for an entity is the entity type.
ER diagram notation for entity student Attribute Each entity has attributes, or particular properties that describe the entity. For example, student Emanuel Vagas has properties of his own Student Identification number, name, and grade.
A particular value of an attribute, such as 93 for the grade, is a value of the attribute. Most of the data in a database consists of values of attributes.
The set of all possible values of an attribute, such as integers from 0 to for a grade, is the attribute domain.
In an ER model, an attribute name appears in an oval that has a line to the corresponding entity box, such as in Figure 3. Definition An attribute of an entity is a particular property that describes the entity. The set of all possible values of an attribute is the attribute domain.
Sometimes the value of an attribute is unknown or missing, and sometimes a value is not applicable. In such cases, the attribute can have the special value of null. For example, until the professor grades a laboratory assignment, the team grade is missing or null.
Definition Null is the special attribute value that indicates an unknown or missing value. An attribute can be simple or composite. 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.An entity relationship diagram (ERD) shows the relationships of entity sets stored in a database.
An entity in this context is an object, a component of data. An entity set is a collection of similar entities. ER Model for Conceptual Design In , Chen developed the Entity-Relationship (ER) model, a high-level data model that is useful in developing a conceptual design for a arteensevilla.comon of an ER diagram, which is one of the first steps in designing a database, helps the designer(s) to understand and to specify the desired components of the database and the relationships among those components.
An entity relationship model, also called an entity-relationship (ER) diagram, is a graphical representation of entities and their relationships to each other, typically used in computing in regard to the organization of data within databases or information systems.
An entity relationship diagram (ERD), also known as an entity relationship model, is a graphical representation of an information system that depicts the relationships among people, objects, places, concepts or events within that system.
An entity relationship model, also called an entity-relationship (ER) diagram, is a graphical representation of entities (which will become your tables) and their relationships to each other.
An entity–relationship model (ER model for short) describes interrelated things of interest in a specific domain of knowledge. A basic ER model is composed of entity types (which classify the things of interest) and specifies relationships that can exist between instances of those entity types.