Entity Relationship Diagram

An entity relationship diagram (ERD) is a representation of data within a domain. It consists of entities as well as relationships between entities.

An entity can be a tangible, physical object such as a school or student, or a concept such as a reply or a transaction. Entity can be identified by extracting objects that are relevant and meaningful to the problem domain and the system to develop. In entity relationship modeling, the term entity has synonyms "table", "database table", "entity-type". Yet, entity is the most commonly used term. Each entity brings along a set of columns, which are the properties of the entity the attributes belong to. For instance, entity Student has name, address and grade as columns (synonyms: attributes, properties, fields). Every entity must have at least one attribute that can be used to uniquely identify the entity, which is known as the entity's primary key(s).

Relationships are capable in linking up entities. Typical examples: one-to-one, one-to-many, many-to-many. The proper use of relationship is important in showing HOW entities are related. For instance, one-to-many relationship must be used for modeling the fact that 'one school has many students'.

Entity Relationship Diagram | Synchronization with Class Diagram

ERD Sample

An entity relationship diagram showing the entities of a simple order processing system.

ERD notations

Here are the notations supported by ERD: Entity, database view, sequence, one-to-one relationship, one-to-many relationship, many-to-many relationship, stored procedure, stored procedure resultset, triggers.

Conceptual ERD

Conceptual ERD outline the primary entities/components of proposed system by drawing a high level design. It is developed by based on the requirements for the system/application to ne developed.

Logical ERD

Logical ERD refines conceptual design to include/exclude primary entities and relate them.

Physical ERD

The most detailed design that is readily adaptable to physical database.

Database-specific column types

An entity (database table) is formed by number of columns which represents the attributes or properties of that entity. Each column contains a column type, which is the type of that property. For example, a column age have type integer. Visual Paradigm supports a number of database types. When you switch between database types, the column types will be updated accordingly, to adopt the new database selection. Here is a list of supported database type: MySQL, MS SQL Server, Oracle, HSQL, Sybase ASE, Sybase SQL Anywhere, PostgreSQL, Cloudscape/Derby, DB2, Ingres, OpenEdge, Informix, Firebird, FrontBase, Cache, SQLite, H2.

Related Links

Tutorials of Visual Paradigm
Visual Paradigm
Know-How - Compare databases with Visual Diff
Know-How - Model sub-type in ERD
Entity Relationship Diagram | Synchronization with Class Diagram
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