What is Entity Relationship Diagram (ERD)?
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. Learn Entity Relationship Diagram (ERD). Read this ERD guide for everything you need to know about data modeling and database design with ERD. This definition explains the meaning of entity relationship diagram, also known as ERD, and how the data model can be used to design relational databases.
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.
The first is the 'fan trap'. It occurs with a master table that links to multiple tables in a one-to-many relationship.
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".
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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.
The ER diagram example below shows an entity with some attributes in it. Primary Key Also known as PK, a primary key is a special kind of entity attribute that uniquely defines a record in a database table. In other words, there must not be two or more records that share the same value for the primary key attribute. The ERD example below shows an entity 'Product' with a primary key attribute 'ID', and a preview of table records in database.
Foreign Key Also known as FK, a foreign key is a reference to a primary key in table.
What is Entity Relationship Diagram (ERD)?
It is used to identify the relationships between entities. Note that foreign keys need not to be unique. Multiple records can share the same values. The ER Diagram example below shows an entity with some columns, among which a foreign key is used in referencing another entity. Relationship A relationship between two entities signifies that the two entities are associated with each other somehow.
For example, student might enroll into a course. The entity Student is therefore related with Course, and the relationships is presented as a connector connecting between them.
Cardinality Cardinality defines the possible number of occurrence in one entity which are associated to the number of occurrences in another. When present in an ERD, the entities Team and Player are inter-connected with a one-to-many relationship.
In an ER diagram, cardinality is represented as a crow's foot at the connector's ends.
Entity–relationship model - Wikipedia
The three common cardinal relationships are one-to-one, one-to-many, and many-to-many. One-to-One cardinality example A one-to-one relationship is mostly used to split an entity in two to provide information concisely and make it more understandable. The figure below shows an example of one-to-one relationship.
One-to-Many cardinality example A one-to-many relationship refers to the relationship between two entities X and Y in which an instance of X may be linked to many instances of Y, but an instance of Y is linked to only one instance of X. The figure below shows an example of one-to-many relationship. Many-to-Many cardinality example A many-to-many relationship refers to the relationship between two entities X and Y in which X may be linked to many instances of Y and vice versa. The figure below shows an example of many-to-many relationship.