Example: set of all persons, companies, trees, holidays ER Model 3. Extended E-R Features Reduction to Relation Schemas Database Design ER Model 2 .. Download ppt "DATABASE MANAGEMENT SYSTEM Entity-Relationship Model. A diagram that displays a set of tables and the relationships between them; Next Slide: “Restaurant” Access data model using Entity Relationship Diagram (ERD). Entity-Relationship Data Model (ERM) is a detailed, logical representation of the data for Entity-Relationship Diagram (ERD) is a graphical representation of a.
The domain is the attribute's set of possible values.
DATABASE MANAGEMENT SYSTEM Entity-Relationship Model. - ppt download
The domain of the attribute "Grade Point Average" is a real number between 0 and 4. The domain of the attribute "Gender" consists of only two possibilities, M or F or some other equivalent code. Attributes may share a domain.
Age is a single valued attribute of the entity person, because a person can have only one age. A composite attribute is a logical grouping of single-valued attributes.
DataBase Data Modeling Using the Entity-Relationship Model - ppt download
A multi-valued attribute may also be composite. An example would be, assuming we allowed multiple phone numbers for an entity, Phone area code, number In an E-R model, Multiple Valued Attributes are shown by a double line or a bold line connecting the attribute to the entity entity class.
Thus, the composite attribute Color will be split into new attributes: Topcolor, Bodycolor and Trimcolor, all linked to the entity class Car. Total cost can be derived by multiplying quantity ordered by unit price.
A Derived Attribute is indicated in an E-R Model by a dotted line connecting the attribute to the entity. In the original Entity-Relationship model.
DataBase Data Modeling Using the Entity-Relationship Model
N" Many-to-Many Abbreviated as "N: No automobile is assigned to more than one employee. A Student will be an occupant of at most ONE dormitory. The positions of the 1 and the N are significant. The 1 refers to the dormitory side of the relationship, and the N refers to the student side of the relationship. A club will have "many" any number of student members.
The numbers inside the relationship diamond show the maximum number of entities which can occur on one side of the relationship. These relationships are often called "HAS A" relationships because, for instance, a student 'has a' dormitory. We will contrast this later with "IS A" relationships.ENTITY RELATIONSHIP DIAGRAM( ER-DIAGRAM)
Maximum cardinality is the maximum number of entity instances that can participate in a relationship. Relationships between more than two entity sets are rare.
DATABASE MANAGEMENT SYSTEM Entity-Relationship Model.
Most relationships are binary. More on this later. Most useful in describing binary relationship sets.
For a binary relationship set the mapping cardinality must be one of the following types: Some elements in A and B may not be mapped to any elements in the other set 13 Keys A super key of an entity set is a set of one or more attributes whose values uniquely determine each entity. ER Model 13 14 Keys for Relationship Sets The combination of primary keys of the participating entity sets forms a super key of a relationship set.
Diamonds represent relationship sets.
We underline the discriminator of a weak entity set with a dashed line. We put the identifying relationship of a weak entity in a double diamond.
A database which conforms to an E-R diagram can be represented by a collection of schemas. For each entity set and relationship set there is a unique schema that is assigned the name of the corresponding entity set or relationship set. Each schema has a number of columns generally corresponding to attributeswhich have unique names.