Data Design ERD diagram - Mr. Ahmad Al-Ghoul
learning Objectives • Describe data relationships • Define cardinality, and use cardinality notation • Draw an entity-relationship diagram
Entity-Relationship Diagrams • What Is an ERD? • A picture showing the information created, stored, and used by a business system. • ERD is network model that describes stored data of a system at a high level of abstraction. • Entities generally represent similar kinds of information • Lines drawn between entities show relationships among the data • High level business rules are also shown
Entity-Relationship Diagrams • Representing entities • we represent an entity by a named rectangle • use a singular noun, or adjective + noun • refer to one instance in naming CUSTOMER PART-TIME EMPLOYEE
Entity-Relationship Diagrams • Entity types and instances • an entity type is a classification of entity instances • Example Ahmad Amal Osama SUPPLIER
Entity-Relationship Diagrams • Relationship • is an association between two entities • we may wish to store information about the association • Relationships should have active verb names • often recognised by a verb or "entity + verb + entity" eg CUSTOMER places ORDER • relationships capture the "business rules" of the system • The first entity in the relationship is the parent entity; the second entity in the relationship is thechild entity • Relationships go in both directions
Entity-Relationship Diagrams • A Relationship Type is a relevant business association between two Entity Types. • A Relationship is an occurrence of a relationship type. Marketing employs Yosef Ali Finance employs Osama Mlkawi MIS employs Ahmad Al ghoul DEPT EMPLOYEE employs employs
Entity-Relationship Diagrams • Types of Relationships • Three types of relationships can exist between entities • One-to-one relationship (1:1): One instance in an entity (parent) refers to one and only one instance in the related entity (child)
Entity-Relationship Diagrams • Types of Relationships • One-to-many relationship (1:M): One instance in an entity (parent) refers to one or more instances in the related entity (child)
Entity-Relationship Diagrams • Types of Relationships • Many-to-many relationship (M:N): exists when one instance of the first entity (parent) can relate to many instances of the second entity (child), and one instance of the second entity can relate to many instances of the first entity.
Entity-Relationship Diagrams • Types of Relationships • Many-to-many relationship (M:N): • this relationship can not represent physically so we solve this problem by dividing it to two (1:M) relationships. • The relationship between STUDENT and CLASS is many-to-many, one student can take many classes, and one class can have many students. • To solve the problem of this relationship note that the event or transaction that links the two entities is actually a third entity, called Associative entity that has its own characteristics. • In the next section (normalization) you will see how we will solve this problem. Associative Entity
Entity-Relationship Diagrams • Cardinalities in relationships • The cardinality of a relationship is the number of instances of one entity type that may be associated with each instance of the other entity type. • An analyst can model this interaction by adding cardinality notation, which uses special symbols to represent the relationship.
Entity-Relationship Diagrams • Cardinality • Cardinality notation: Notation that shows relationships between entities. • Crow’s foot notation: A type of cardinality notation. It is called crow's foot notation because of the shapes, which include circles, bars, and symbols, that indicate various possibilities. A single bar indicates one, a double bar indicates one and only one, a circle indicates zero, and a crow's foot indicates many.
Entity-Relationship Diagrams Crow's foot notation is a common method of indicating cardinality. The four examples show how you can use various symbols to describe the relationships between entities.
Entity-Relationship Diagrams In the first example of cardinality notation, one and only one CUSTOMER can place anywhere from zero to many of the ORDER entity. In the second example, one and only one ORDER can include one ITEM ORDERED or many. In the third example, one and only one EMPLOYEE can have one SPOUSE or none. In the fourth example, one EMPLOYEE, or many employees, or none, can be assigned to one PROJECT, or many projects, or none.
Entity-Relationship Diagrams • The entity-relationship diagram may be used to determine record keys • When the relationship is one-to-many, the primary key of the file at the one end of the relationship should be contained as a foreign key on the file at the many end of the relationship • A many-to-many relationship should be divided into two one-to-many relationships with an associative entity in the middle
Sequence Summary • An entity-relationship diagram (ERD) is a graphic representation of all system entities and the relationships among them • The ERD is based on entities and data stores in DFDs prepared during the systems analysis phase • The three basic relationships represented in an ERD are one-to-one (1:1), one-to-many (1:M), and many-to-many (M:N) • In a M:N relationship, the two entities are linked by an associative entity • The relationship between two entities also is referred to as cardinality • A common form if cardinality notation is called crow’s foot notation, which uses various symbols to describe the characteristics of the relationship
Sequence Summary • In this Sequence we have • Defined entity relationship diagram (ERD) • Defined and distinguished between entity type and entity instance • Defined and distinguished between relationship and relationship type • Explained types of relationships • Defined cardinality, and used cardinality notation
Reference  System Analysis and Design, Sixth Edition Authors: Gary B. Shelly, Thomas J. Cashman and Harry J. Rosenblatt Publisher: SHELLY CASHMAN SEWIES.