Entity–relationship model - Wikipedia
The degree of relationship (also known as cardinality) is the number of occurrences in one entity which are associated (or linked) to the number of occurrences. ER Model or Entity-relationship model is a database model based on entity, is an Entity, then the complete dataset of all the students will be the Entity Set. Relationship in DBMS- Before you go through this article, make sure that you have gone through the previous article on Introduction to ER Diagrams.Relationship in DBMS in hindi - Degree,Cardinality,Participation - DBMS gate lectures in hindi - #5
The UML specification explicitly states that associations in class models are extensional and this is in fact self-evident by considering the extensive array of additional "adornments" provided by the specification over and above those provided by any of the prior candidate "semantic modelling languages". It incorporates some of the important semantic information about the real world. Plato himself associates knowledge with the apprehension of unchanging Forms The forms, according to Socrates, are roughly speaking archetypes or abstract representations of the many types of things, and properties and their relationships to one another.
Limitations[ edit ] ER assume information content that can readily be represented in a relational database. They describe only a relational structure for this information.
They are inadequate for systems in which the information cannot readily be represented in relational form[ citation needed ], such as with semi-structured data. For many systems, possible changes to information contained are nontrivial and important enough to warrant explicit specification. An alternative is to model change separately, using a process modeling technique. Additional techniques can be used for other aspects of systems. For instance, ER models roughly correspond to just 1 of the 14 different modeling techniques offered by UML.
Even where it is suitable in principle, ER modeling is rarely used as a separate activity. One reason for this is today's abundance of tools to support diagramming and other design support directly on relational database management systems.
The Entity-Relationship Model
These tools can readily extract database diagrams that are very close to ER diagrams from existing databases, and they provide alternative views on the information contained in such diagrams.
In a survey, Brodie and Liu  could not find a single instance of entity—relationship modeling inside a sample of ten Fortune companies.
Badia and Lemire  blame this lack of use on the lack of guidance but also on the lack of benefits, such as lack of support for data integration. To ensure that you get a precise understanding of the nature of the data and how the enterprise uses it, you need to have a universal model for interaction that is non-technical and free of ambiguities and easily readable to both technical as well as non-technical members.
In this chapter, you will learn about the ER diagram and its working. ER-Diagram is a pictorial representation of data that describes how data is communicated and related to each other.
- Basic Concepts of ER Model in DBMS
- Working with ER Diagrams
- Entity Relationship(ER) Model
Any object, such as entities, attributes of an entity, sets of relationship and other attributes of relationship can be characterized with the help of the ER diagram. They are represented using the rectangle shape box. These rectangles are named with the entity set they represent.
relationship set in dbms with example | Gate Vidyalay
ER modeling is a top-down structure to database design that begins with identifying the important data called entities and relationships in combination with the data that must be characterized in the model. Then database model designers can add more details such as the information they want to hold about the entities and relationships which are the attributes and any constraints on the entities, relationships, and attributes.
ER modeling is an important technique for any database designer to master and forms the basis of the methodology.
It is a group of objects with the same properties that are identified by the enterprise as having an independent existence. The basic concept of the ER model is the entity type that is used to represent a group of 'objects' in the 'real world' with the same properties.
Let us consider another scenario, where we want to store the information of employees and their dependents. The every employee may have zero to n number of dependents. Every dependent has an id number and name. Now let us consider the following data base: There are three employees having E as 1, 2, and 3 respectively. Employee having E 1, has two dependents as 1, Rahat and 2, Chahat.
Relationships & Relationship Sets
Employee having E 2, has no dependents. Employee having E 3, has three dependents as 1, Raju; 2, Ruhi; 3 Raja. Now, in case of Dependent entity id cannot act as primary key because it is not unique.
Thus, Dependent is a weak entity set having id as a discriminator. It has a total participation with the relationship "has" because no dependent can exist without the employees the company is concerned with employees.