What is Entity Relationship Diagram (ERD)?
Peter Chen developed ERDs in Since then Charles Bachman and James Martin have added some slight refinements to the basic ERD principles. ERDPlus is a database modeling tool to quickly and easily create Entity Relationship Diagrams, Relational Schemas, and Star Schemas. An entity–relationship model (ER model for short) describes interrelated things of interest in a . Entity–relationship diagrams don't show single entities or single instances of relations. Rather, they show entity sets (all entities of the same entity .
Model usability issues[ edit ] You can help by adding to it. February In using a modeled database, users can encounter two well known issues where the returned results mean something other than the results assumed by the query author.
- Navigation menu
- What is an ER diagram (ERD)?
The first is the 'fan trap'. It occurs with a master table that links to multiple tables in a one-to-many relationship.
Entity–relationship model - Wikipedia
The issue derives its name from the way the model looks when it's drawn in an entity—relationship diagram: This type of model looks similar to a star schemaa type of model used in data warehouses.
When trying to calculate sums over aggregates using standard SQL over the master table, unexpected and incorrect results. The solution is to either adjust the model or the SQL.
This issue occurs mostly in databases for decision support systems, and software that queries such systems sometimes includes specific methods for handling this issue. The second issue is a 'chasm trap'. A chasm trap occurs when a model suggests the existence of a relationship between entity types, but the pathway does not exist between certain entity occurrences.
For example, a Building has one-or-more Rooms, that hold zero-or-more Computers. One would expect to be able to query the model to see all the Computers in the Building. However, Computers not currently assigned to a Room because they are under repair or somewhere else are not shown on the list. Another relation between Building and Computers is needed to capture all the computers in the building. This last modelling issue is the result of a failure to capture all the relationships that exist in the real world in the model.
See Entity-Relationship Modelling 2 for details. Entity—relationships and semantic modeling[ edit ] Semantic model[ edit ] A semantic model is a model of concepts, it is sometimes called a "platform independent model". 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". An ERD contains different symbols and connectors that visualize two important information: The major entities within the system scope, and the inter-relationships among these entities.
Studenttangible business objects e. Productintangible business objects e. In a typical ER design, you can find symbols such as rounded rectangles and connectors with different styles of their ends that depict the entities, their attributes and inter-relationships.
When to draw ER Diagrams? So, when do we draw ERDs? While ER models are mostly developed for designing relational database in terms of concept visualization and in terms of physical database design, there are still other situations when ER diagrams can help. Here are some typical use cases.
Database design - Depending on the scale of change, it can be risky to alter a database structure directly in a DBMS. To avoid ruining the data in a production database, it is important to plan out the changes carefully.
ERD is a tool that helps. By drawing ER diagrams to visualize database design ideas, you have a chance to identify the mistakes and design flaws, and to make correction before executing the changes in database.
Database debugging - To debug database issues can be challenging, especially when the database contains many tables, which require writing complex SQL in getting the information you need. By visualizing a database schema with an ERD, you have a full picture of the entire database schema.
You can easily locate entities, view their attributes and to identify the relationships they have with others.
All these allows you to analyze an existing database and to reveal database problem easier. Database creation and patching - ERD tool like Visual Paradigm supports database generation tool that can automate the database creation and patching process by means of ER diagrams. So, with this ER Diagram tool your ER design is no longer just a static diagram but a mirror that reflects truly the physical database structure.
Aid in requirements gathering - Determine the requirements of an information system by drawing a conceptual ERD that depicts the high-level business objects of the system. Such an initial model can also be evolved into physical database model that aids the creation of relational database, or aids in the creation of process map and data flow model.
In this section we will go through the ERD symbols in detail. Studentobject e. Invoiceconcept e. Profile or event e.
ER Diagram Representation
In ERD, the term "entity" is often used instead of "table", but they are the same. When determining entities, think of them as nouns. In ER models, an entity is shown as a rounded rectangle, with its name on top and its attributes listed in the body of the entity shape.
Entity Attributes Also known as column, an attribute is a property or characteristic of the entity that holds it. An attribute has a name that describes the property and a type that describes the kind of attribute it is, such as varchar for a string, and int for integer.