January 29, 2020
Amitraj
Mapping Constraints -
A mapping constraint is a data constraint that expresses the number of entities to which another entity can be related via a relationship set.
It is most useful in describing the relationship sets that involve more than two entity sets.
Mapping Cardinalities -
Mapping constraints can be explained in terms of mapping cardinality.
Cardinality defines the number of entities in one entity set, which can be associated with the number of entities of other set via relationship set.
1. One to one (1:1)
2. One to many (1:M)
3.Many to one (M:1)
4. Many to many (M:M)
1. One-to-one − One entity from entity set A can be associated with at most one entity of entity set B and vice versa.
A mapping constraint is a data constraint that expresses the number of entities to which another entity can be related via a relationship set.
It is most useful in describing the relationship sets that involve more than two entity sets.
Mapping Cardinalities -
Mapping constraints can be explained in terms of mapping cardinality.
Cardinality defines the number of entities in one entity set, which can be associated with the number of entities of other set via relationship set.
1. One to one (1:1)
2. One to many (1:M)
3.Many to one (M:1)
4. Many to many (M:M)
1. One-to-one − One entity from entity set A can be associated with at most one entity of entity set B and vice versa.
2. One-to-many − One entity from entity set A can be associated with more than one entities of entity set B however an entity from entity set B, can be associated with at most one entity.
3. Many-to-one − More than one entities from entity set A can be associated with at most one entity of entity set B, however an entity from entity set B can be associated with more than one entity from entity set A.
ER Model: Keys
Key is an attribute or collection of attributes that uniquely identifies an entity among entity set.
If the attribute roll no. can uniquely identify a student entity, amongst all the students, then the attribute roll no. will be said to be a key.
Following are the types of Keys:
Super Key − A set of attributes (one or more) that collectively identifies an entity in an entity set.
Candidate Key − A minimal super key is called a candidate key. An entity set may have more than one candidate key.
Primary Key − A primary key is one of the candidate keys chosen by the database designer to uniquely identify the entity set.
4. Many-to-many − One entity from A can be associated with more than one entity from B and vice versa.
ER Model: Keys
Key is an attribute or collection of attributes that uniquely identifies an entity among entity set.
If the attribute roll no. can uniquely identify a student entity, amongst all the students, then the attribute roll no. will be said to be a key.
Following are the types of Keys:
Super Key − A set of attributes (one or more) that collectively identifies an entity in an entity set.
Candidate Key − A minimal super key is called a candidate key. An entity set may have more than one candidate key.
Primary Key − A primary key is one of the candidate keys chosen by the database designer to uniquely identify the entity set.
