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ERD Design, Keys, and Attributes in Database Systems

This text provides an overview of the principles for designing entity-relationship diagrams (ERDs) in database systems. It covers topics such as keys, attributes, and relationships, and discusses how these components are represented in tables.

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ERD Design, Keys, and Attributes in Database Systems

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  1. Chapters 3 and 4Drawing ERDs October 16, 2006 Week 3

  2. M EMPLOYEE 1 manages is assigned to 1 1 M 1 is composed of DIVISION DEPARTMENT Practice … (correction) • Each of the MegaCo Corporation’s divisions is composed of many departments. Each of those departments has many employees assigned to it, but each employee works for only one department. Each department is managed by one employee, and each of those managers can manage only one department at a time. Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  3. A little from Chapters 3 and 4 • Good design begins by identifying appropriate entities and attributes and the relationships among the entities • Chapter 3: Section 3.2 Keys (pages 64-69) • Chapter 4 All Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  4. The Entity Relationship Diagram (ERD) • ERD represents the conceptual database as viewed by end user • ERDs depict the ER model’s three main components: • Entities - anything about which data are to be collected and stored (i.e. people, events) • Attributes - characteristics of an entity • Relationships - describe an association among (two or more) entities (1:1, 1:M, M:N) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  5. Tables and Keys • Tables are basic building blocks of a relational database • Keys are central to the use of relational tables Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  6. Tables and Their Characteristics Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  7. Keys • Consists of one or more attributes that determine other attributes • Primary key (PK) is an attribute (or a combination of attributes) that uniquely identifies any given row • Composite key - Composed of more than one attribute • Foreign key (FK) • An attribute whose values match primary key values in the related table • Key’s role is based on determination • If you know the value of attribute A, you can look up (determine) the value of attribute B Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  8. Keys (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  9. Keys (continued) • Nulls: • No data entry • Not permitted in primary key • Should be avoided in other attributes • Can represent • An unknown attribute value • A known, but missing, attribute value • A “not applicable” condition • Can create problems when functions such as COUNT, AVERAGE, and SUM are used • Can create logical problems when relational tables are linked Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  10. Keys (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  11. Entities and Instances • Each entity ultimately becomes a table in a database implementation • Each instance of an entity is a record or a row in a table in a database • Entity name, a noun, is usually written in capital letters Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  12. Attributes • Attributes ultimately become the fields in a database implementation • Attributes are represented by ovals and are connected to entity rectangle with a line • Primary Key is underlined in the ERD Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  13. Types of Attributes • Key attribute is a unique identifier • Composite attribute can be subdivided • Simple attribute cannot be subdivided • Single-value attribute can have only a single value • Multivalued attributes can have many values • A student may have more that one phone number; a home-phone number and a mobile-phone number • Derived attribute’s value may be calculated (derived) from other attributes • Need not be physically stored in a database Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  14. Multivalued Attributes Represented by a double line Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  15. Resolving Multivalued Attribute Problems • Although conceptual model can handle M:N relationships and multivalued attributes, you should not implement them in relational DBMS • Within original entity, create several new attributes, one for each of the original multivalued attribute’s components • Can lead to major structural problems in table • Create new entity composed of original multivalued attribute’s components Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  16. Resolving Multivalued Attribute Problems (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  17. Derived Attributes (continued) Represented by a dashed line Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  18. Connectivity and Cardinality • Connectivity • Used to describe the relationship classification (based on participation) • 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: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  19. M N is assigned to EMPLOYEE PROJECT Cardinality Connectivity Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  20. 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: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  21. 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: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  22. Weak (Non-Identifying) Relationships Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  23. Strong (Identifying) Relationships Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  24. Weak Entities • Weak entity meets two conditions • Existence-dependent • Cannot exist without entity with which it has a relationship • Has primary key that is partially or totally derived from parent entity in relationship • Database designer usually determines whether an entity can be described as weak based on business rules Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  25. Weak Entities (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  26. 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: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  27. Relationship Degree (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  28. Composite Entities • Also known as bridge entities • Composed of primary keys of each of the entities to be connected • May also contain additional attributes that play no role in connective process • Used to resolve M:N relationships Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  29. Mandatory participation 1, N, M Source: http://io.uwinnipeg.ca/~rmcfadye/2914/ERD/images/1entit4.gif Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  30. Practice ... • An airplane has a registration number, type, number of economy class seats, number of business class seats and the year of production Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  31. Practice ... • Each text book has a unique ISBN (International Standard Book Number), and contains several chapters. Each chapter has a chapter number (unique within a book), the number of pages and the number of references. A chapter covers a single topic, but the same topic may be covered in various books. Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  32. Practice ... • Sometimes students work in groups. Each group has a unique number and students have their student ids. A student who works in a group has a specific role within that group. The student may have different roles in various groups he/she belongs to. Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

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