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Sunday, August 9, 2009

Relationships And Relationship Sets

A relationship is an association between several entities.
A relationship set is a set of relationships of the same type.
A relationship type R among n entity types E1, E2,....., En defines a set of associations or a relationship set among entities from these types.
In ER diagrams, relationship types are displayed as diamond-shaped boxes, which are connected by straight lines to the rectangular boxes representing the participating entity types. The relationship name is displayed in the diamond-shaped box.

RELATIONSHIP DEGREE : The degree of a relationship type is the number of participating entity types. A relationship type of degree two is called binary, and one with degree three is called ternary.

Binary Degree
Ternary Degree

Role names signifies the role that a participating entity from the entity type plays in each relationship instance, and helps to explain what the relationship means.
Role names are not necessary in relationship types where all the participating entity types are distinct, since each entity type name can be used as a role name. However, in some cases, the same entity type participates more than once in a relationship type in different roles. In such cases the role name becomes essential for distinguishing the meaning of each participation. Such relationships are called recursive relationships.

An E-R scheme may define certain constraints to which the contents of a database must conform.
* Mapping Cardinality : It expresses the number of entities to which another entity can be associated via a relationship. For binary relationship sets between entity sets A and B, the mapping cardinality must be one of:
1. One-to-one: An entity in A is associated with at most one entity in B, and an entity in B is associated with at most one entity in A.
2. One-to-many: An entity in A is associated with any number in B. An entity in B is associated with at most one entity in A.
3. Many-to-one: An entity in A is associated with at most one entity in B. An entity in B is associated with any number in A.
4. Many-to-many: Entities in A and B are associated with any number from each other.
The appropriate mapping cardinality for a particular relationship set depends on the real world being modeled.
* Existence Dependencies: if the existence of entity X depends on the existence of entity Y, then X is said to be existence dependent on Y. (Or we say that Y is the dominant entity and X is the subordinate entity.)
For example,
o Consider account and transaction entity sets, and a relationship log between them.
o This is one-to-many from account to transaction.
o If an account entity is deleted, its associated transaction entities must also be deleted.
o Thus account is dominant and transaction is subordinate.

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