CHAPTER 3 ENTITY RELATIONSHIP ER MODELLING PREPARED BY DR MARJAN BINTI MOHD NOOR LEARNING OBJECTIVES On completion of this chapter you should be able to The main characteristics of entity relationship components How relationships between entities are defined refined and incorporated into the database design process How ERD components affect database design and implementation That real world database design often requires the reconciliation of conflicting goals THE ENTITY RELATIONSHIP ER MODEL ER model forms the basis of an ER diagram ERD represents conceptual database as viewed by end user ERDs depict database s main components Entities Attributes Relationships ENTITIES Refers to entity set and not to single entity occurrence In Chen and Crow s Foot models entity represented by rectangle with entity s name Corresponds to table and not to row in relational environment Entity name a noun written in capital letters ATTRIBUTES CHARACTERISTICS OF ENTITIES CHEN NOTATION Attributes represented by ovals connected to entity rectangle with a line Each oval contains the name of attribute it represents CROW S FOOT NOTATION Attributes written in attribute box below entity rectangle ATTRIBUTES continued REQUIRED ATTRIBUTES Must have a value 1 COMPOSITE IDENTIFIERS Primary key composed of more than one attribute 5 OPTIONAL ATTRIBUTES May be left empty 2 DOMAIN Set of possible values for an attribute IDENTIFIERS One more attributes that uniquely identify each entity instance 4 3 Before deleting After deleting ATTRIBUTES continued Composite Simple attribute attribute can have only a Multivalued single value attributes can be subdivided cannot be subdivided simple serial can have many values Single value attribute Exm age number composite SE 08 02 189935 ATTRIBUTES continued M N relationships and multivalued attributes should not be implemented Create several new attributes for each of the original multivalued Create new entity composed of original multivalued attributes attributes components components Derived attribute value may be calculated from other attributes Need not be physically stored within database RELATIONSHIPS Association between entities Participants are entities that participate in a relationship Relationships between entities always operate in both directions Relationship can be classified as 1 M Relationship classification is difficult to establish if only one side of the relationship is known CONNECTIVITIES Connectivity Describes the relationship classification Cardinality Expresses minimum and maximum number of entity occurrences associated with one occurrence of related entity Established by very concise statements known as business rules CONNECTIVITIES CARDINALITY 1 N Exm no upper limit to the number of classes a professor might teach Existence Dependence Existence dependence Entity exists in database only when it is associated with another related entity occurrence Existence independence Entity can exist apart from one or more related entities Sometimes such an entity is referred to as a strong or regular entity Relationship Strength Weak non identifying relationships Exists if PK of related entity does not contain PK component of parent entity COURSE CRS CODE DEPT CODE CRS DESCRIPTION CRS CREDIT CLASS CLASS CODE CRS CODE CLASS SECTION CLASS TIME ROOM CODE PROF NUM Relationship Strength Strong identifying relationships Exists when PK of related entity contains PK component of parent entity COURSE CRS CODE DEPT CODE CRS DESCRIPTION CRS CREDIT CLASS CRS CODE CLASS SECTION CLASS TIME ROOM CODE PROF NUM Weak Entities Weak entity meets two conditions Existence dependent Primary key partially or totally derived from parent entity in relationship Database designer determines whether an entity is weak based on business rules EMPLOYEE EMP NUM EMP LNAME EMP FNAME EMP INITIAL EMP DOB EMP HIREDATE DEPENDENT EMP NUM DEP NUM DEP FNAME DEP DOB Relationship Participation Optional participation One entity occurrence does not require corresponding entity occurrence in particular relationship Mandatory participation One entity occurrence requires corresponding entity occurrence in particular relationship Relationship Degree Indicates number of entities or participants associated with a relationship Unary relationship Association is maintained within single entity Binary relationship Two entities are associated Ternary relationship Three entities are associated Recursive Relationships Relationship can exist between occurrences of the same entity set Naturally found within unary relationship Associative Composite Entities Also known as bridge entities Used to implement M N relationships Composed of primary keys of each of the entities to be connected May also contain additional attributes that play no role in connective process Developing an ER Diagram Database design is an iterative process Create detailed narrative of organization s description of operations Identify business rules based on description of operations Identify main entities and relationships from business rules Develop initial ERD Identify attributes and primary keys that adequately describe entities Revise and review ERD Database Design Challenges Conflicting Goals Database designers must make design compromises Conflicting goals design standards processing speed information requirements Important to meet logical requirements and design conventions Design of little value unless it delivers all specified query and reporting requirements Some design and implementation problems do not yield clean solutions Summary Entity relationship ER model Uses ERD to represent conceptual database as viewed by end user ERM s main components Entities Relationships Attributes Includes connectivity and cardinality notations Summary continued Connectivities and cardinalities are based on business rules M N relationship is valid at conceptual level Must be mapped to a set of 1 M relationships ERDs may be based on many different ERMs Database designers are often forced to make design compromises THANK YOU