Understanding ERD
In software engineering, an Entity-Relationship Model (ERM) is an abstract and conceptual representation of the data. Entity-Relationship modeling method is one of the data base used to generate the conceptual scheme for the type / semantic data model system. Where the system often have a relational database, and its provisions are top-down. The diagram to illustrate the model is called the entitiy-Relationship-Relationship diagrams entitiy, ER diagrams, or ERD.
ERD notation
There are a number of conventions on ERD notation. Classical notation is often used for conceptual models. Various other notations are also used to describe logically and physically from a database, one of which is IDEF1X.
Symbolic notations used in the Entity Relationship Diagram is as follows:
Entity, is everything that can be described by the data. Entities can also be interpreted as an individual representing something real (existence) and can be distinguished from something else (Fathansyah, 1999). There are two kinds of entities, namely a strong entity and the entity is weak. Strong entity is an entity that does not have a dependency with other entities. Examples entity members. While the weak entity is an entity whose occurrence depends on the existence of other entities in a relation.
Attribute, Attributes is a description of the characteristics of the entity. Attributes depicted in the form of a circle or ellipse. Key attributes of the entity or the key is underlined.
Relations or relations, relations indicate a relationship between the number of entities that come from a different set of entities.
Liaison between the relationship set and the entity set entity set with attributes expressed in the form of a line.
Degree relatives or cardinality
Indicates the maximum number of entities that can relate to entities on the set of another entity. Various cardinality is:
One-to-one (one to one), each member of the entity A may only be associated with one member of the entity B, and vice versa.
One to many (one to many), each member of the entity A may be associated with more than one member of the entity B but not vice versa.
Many to many (many to many), each entity A may be associated with many entities entity set B and vice versa.
ERD stage
The first stage in the design of information systems using the ER model is to describe information needs or the type of information to be stored in the database. Data modeling techniques can be used to describe any ontology (ie an overview and classification of terms used and the relationship of information) for a particular region.
The next stage is called logical design, where data is mapped to the logical data model, such as the relational model. Loguis data model is then mapped into the physical model, so that sometimes, the second phase is referred to as "physical design".
In general methodology ERD as follows:
Sample Case:
A company has several parts. Each piece has at least one supervisor and employee. Employee assigned at least one section (may also in some parts). At least one employee has the task in one project. But the holiday and an employee can not be the task in the project.
Determining entity
Entity: supervisor, parts, employees, projects
Determining the relationship with matrix relation
ERD images while
Connect entities in accordance with the relation matrix created
Filling cardinality
From the description of the problem can be seen that:
each section has only one supervisor
a supervisor on duty in one piece
each section there is at least one employee
each employee working at least in one part
each project undertaken by at least one employee
determine the key
The key point: The number of Trustees, Section Name, Employee Number, Project Number.
Drawing ERD based key
There are two many-to-many relationships in the ERD interim, between sections with employees, employees with a project, therefore we create a new entity that is part -pegawai and employee-project A key element of the new entity is the primary key of another entity that will be the key Guests in the new entity.
Specify attributes
Required attributes are: name section, the project name, employee name, supervisor name, project number, number of employees, number of inspectors
Mapped attributes
Section: Section name
Project: Project name
Employees: employee name
Supervisor: Name of supervisor
Project-Employee: Number of projects, number of employees
Supervisor: Number watchdog
Drawing ERD with attributes
Checking Results
Check if there are still redundasi. ERD end: for modeling data on the system.
