Entity and entity set definition in dbms | entity and entity type in dbms | Entity, Entity Type, set
Entity and entity set definition in dbms | entity and entity type in dbms | Entity, Entity Type, set:
What are we going to learn?
What is an entity?
What is an entity type?
What is an entity set?
What is entity?
Entities are the real world objects or a thing in a real world which have their independent existence.
Now an entity can have a physical existence or conceptual existence.
What is an entity type?
A database usually contains groups of entities that are similar. For example, a company employing hundreds of employees may want to store similar information concerning each of the employees.
These employee entities share the same attributes, but each entity has its own value(s) for each attribute.
An entity type defines a collection (or set) of entities that have the same attributes.
Each entity type in the database is described by its name and attributes.
What is Entity Set?
The collection of all entities of a particular entity type in the database at any point in time is called an entity set.
The entity set is usually referred to using the same name as the entity type.
Example
Representation of Entity type
An entity type is represented in an ER diagram with a rectangular box, enclosing entity type name.
Attribute names are enclosed in an oval and are connected to the entity type with a straight line.
Composite attributes are attached to their component attributes with straight lines.
Multivalued attributes are represented with double ovals enclosing the attribute name.
Example
Key attributes of Entity type
An important constraints of entities of entity types, is uniqueness constraints or key of the attributes.
An entity type has one or more attributes that have unique value for each entity of the entity set.
Such an attribute is called key attribute.
Key attributes can be used to identify each entity uniquely.
For example Vehicle id for each car is different. Hence it’s a key attribute for the entity “car”.
Similarly composite attributes can also be key attributes, like registration number, which is formed of two component attributes, state and number which are not unique on it’s own, but a composite attribute made from both of these two attributes combined form an unique attribute or also known as key attribute of entity type.
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