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Database Systems: Design, Implementation, and Management Tenth Edition
Chapter 4 Entity Relationship (ER) Modeling
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4.1 The Entity Relationship Model (ERM)
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 Database Systems, 10th Edition
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Entities Refers to entity set and not to single entity occurrence
Corresponds to table and not to row in relational environment In Chen and Crow’s Foot models, entity is represented by rectangle with entity’s name The entity name, a noun, is written in capital letters Database Systems, 10th Edition
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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 Database Systems, 10th Edition
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Database Systems, 10th Edition
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Attributes (cont’d.) Required attribute: must have a value
Optional attribute: may be left empty Domain: set of possible values for an attribute For example, the domain for the grade point average (GPA) attribute is written (0,4) because the lowest possible GPA value is 0 and the highest possible value is 4. Identifiers: one or more attributes that uniquely identify each entity instance Composite identifier: primary key composed of more than one attribute Database Systems, 10th Edition
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Identifiers (primary keys)
Identifiers: one or more attributes that uniquely identify each entity instance Composite identifier: primary key composed of more than one attribute Database Systems, 10th Edition
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Database Systems, 10th Edition
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Database Systems, 10th Edition
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Attributes (cont’d.) Composite attribute can be subdivided
Simple attribute cannot be subdivided Database Systems, 10th Edition
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Attributes (cont’d.) Single-value attribute can have only a single value Multivalued attributes can have many values Database Systems, 10th Edition
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Attributes (cont’d.) Database Systems, 10th Edition
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Attributes (cont’d.) M:N relationships and multivalued attributes should not be implemented Create new entity composed of original multivalued attributes’ components Database Systems, 10th Edition
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Database Systems, 10th Edition
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Attributes (cont’d.) A derived attribute is an attribute whose value is calculated (derived) from other attributes. The derived attribute need not be physically stored within the database; instead, it can be derived by using an algorithm. Database Systems, 10th Edition
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For example, an employee’s age, EMP_AGE, may be found by computing the integer value of the difference between the current date and the EMP_DOB. If you use Microsoft Access, you would use the formula INT((DATE() – EMP_DOB)/365). Database Systems, 10th Edition
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Database Systems, 10th Edition
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Connectivity and Cardinality
Describes the relationship classification: 1:1 or 1:M or M:N 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 Database Systems, 10th Edition
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Database Systems, 10th Edition
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Existence Dependence Existence dependence Existence independence
means that an instance of one entity cannot exist without the existence of some other related entity. Ex: Transactions are existence dependent on accounts. Existence independence Entity can exist apart from one or more related entities Database Systems, 10th Edition
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Relationship Strength
Weak (non-identifying) relationships Exists if PK of related entity does not contain PK component of parent entity Strong (identifying) relationships Exists when PK of related entity contains PK component of parent entity Database Systems, 10th Edition
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Database Systems, 10th Edition
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Database Systems, 10th Edition
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Database Systems, 10th Edition
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Database Systems, 10th Edition
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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 Database Systems, 10th Edition
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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 Database Systems, 10th Edition
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Optional participation
For example, in the “COURSE generates CLASS” relationship, at least some courses do not generate a class. In other words, an entity occurrence (row) in the COURSE table does not necessarily require the existence of a corresponding entity occurrence in the CLASS table. Therefore, the CLASS entity is considered to be optional to the COURSE entity Database Systems, 10th Edition
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Database Systems, 10th Edition
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Distinction between mandatory or optional
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1- Class is optional 2- Class is mandatory
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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 Database Systems, 10th Edition
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Database Systems, 10th Edition
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Database Systems, 10th Edition
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Recursive Relationships
Relationship can exist between occurrences of the same entity set Naturally found within unary relationship Database Systems, 10th Edition
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Recursive Relationships
1:1 unary relationship may be expressed by “an EMPLOYEE may be married to one and only one other EMPLOYEE.” Database Systems, 10th Edition
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Recursive Relationships
the 1:M recursive relationship “EMPLOYEE manages EMPLOYEE,” is implemented in Figure 4.22. Database Systems, 10th Edition
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Associative (Composite) Entities
Also known as bridge entities Used to implement M:N relationships. You should recall from Chapter 3 that the relational model generally requires the use of 1:M relationships. If M:N relationships are encountered, you must create a bridge between the entities that display such relationships. The associative entity is used to implement a M:N relationship between two or more entities. Database Systems, 10th Edition
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Associative (Composite) Entities
This associative entity (also known as a composite or bridge entity) composed of the primary keys of each of the entities to be connected. An example of such a bridge is shown in Figure 4.23. May also contain additional attributes that play no role in connective process Database Systems, 10th Edition
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Database Systems, 10th Edition
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Database Systems, 10th Edition
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4. 2 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 Systems, 10th Edition
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Database Systems, 10th Edition
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Database Systems, 10th Edition
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Database Systems, 10th Edition
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Database Systems, 10th Edition
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Database Systems, 10th Edition
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Database Systems, 10th Edition
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initially exist (at the start of the enrollment period) even though no students have enrolled in it, STUDENT is optional to CLASS in the M:N relationship. This M:N relationship must be divided into two 1:M relationships through the use of the ENROLL entity, shown in the ERD segment in Figure 4.31. Database Systems, 10th Edition
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Database Systems, 10th Edition
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Database Systems, 10th Edition
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Figure 4.37 shows the implementation-ready UML class diagram for Tiny College (note that the ENROLL composite entity is shown in this class diagram. Database Systems, 10th Edition
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