Entities and Entity Sets
Summary of Relational and Object-Oriented Database Differences. from a collection of functions performed on data to a collection of abstract. Relational. DBMS. • Entity-Relationship model is used in the conceptual design of a database Roles: The same entity type can participate more than once in a . of entity, attribute, and entity type reflect the technical capabilities of records by using linkages and then is stored in the MDM database.
An order is made An order is made An order is made … and a third file to keep the list of items ordered. An item is ordered An item is ordered … Quite useless, right? However, there still is some information, as per request.
We know that each row in the files describe a customer, an order or an item, and we can get the total number of customers, orders and items we've had so far. This is better than nothing but still far from being usable.Extended ER Features, Generalization, Specialization, Aggregation in DBMS with Example
Note that the diagram tells us nothing of how they should be stored. We could write them into a file, or on a paper or carve it on a stone.
Relationships Diamonds mean relationships. How the entities are related to each other? We see that we should store the relationships between orders and clients as well as those between items and orders.
What is Entity Relationship Diagram? Webopedia Definition
In our cases, we have a 1: Want to store an order? Be prepared to link it to the piece of information about the client who made it and make sure this information describes exactly one client. This is the database developer's task. However, a client can have an arbitrary number of orders: The database should provide an ability to store the clients and orders this way. Again, the entity-relationship model does not prescribe how should we store that data, as long as the storage method satisfies the conditions above.
We could store it in two files and relate them using the row numbers: A customer has registered A customer has registered A customer has registered … An order is made by the customer described on line 1 An order is made by the customer described on line 2 An order is made by the customer described on line 2or we can just keep the information in a single text file: The latter one limits us to only 2 clients and 5 orders, but, you know, every system has its limitations.
Attributes Ovals are attributes.
Key concepts: Entity, attribute, and entity type
What information is stored in the database? Relationship E-R Model is easier to understand the relationship between entities. Comparatively, it is less easy to derive a relation between tables in Relational Model.
Relational Model does not describe mapping cardinalities. E-R Model can be expressed as the collection of entities, also called as real word objects and relations between those entities. No two entities should be identical. E-R Model shows the conceptual view of the database.
E-R Model is formed by the entity set, relationship set and the attributes. Where all the entities collectively form entity set, all the relations between the entities collectively form relationship set, and the attribute describes the properties of entities.
It can be explained with an example. Let us take two real-world entities Employ and Department it will form an Entity set. Now we can easily easy derive a relation between these two entities that an Employ works-for a Department. A rectangle represents an entity in the ER diagram. A double rectangle represents a weak entity while n oval represents the attributes of an entity.
Refer the below diagram.
They are also called strong entities as they do not depend on other entities. The Lecturer entity has attributes id, name, and specialty.
The Student entity has the id and name attribute. The Exam entity depends on the Course Entity. Therefore, Exam is a weak entity.
There are three types of relationships that can exist between the entities. They are the binary, recursive and the ternary relationship. It is further classified into three types. They are one to one relationship 1: When one instance of an entity is associated with one instance of another type of entity, it is called one to one 1: There is one Head of the Department to manage the department.
So, it is a one to one relationship. When one instance of an entity is associated with many instances of another type of entity, it is called one to many 1: