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Entity Relationships

Entity Relationships. A337. Data Stores-Specific Diagrams. Included on both Flowcharts and DFDs There are also rules related to data stores. For example Can there be multiple invoices with the same number? Can a customer have more than one address?

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Entity Relationships

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  1. Entity Relationships A337

  2. Data Stores-Specific Diagrams • Included on both Flowcharts and DFDs • There are also rules related to data stores. For example • Can there be multiple invoices with the same number? • Can a customer have more than one address? • Can an address belong to more than one customer?

  3. Relationships within the Relational Database Maximum Cardinality (the “outside” symbols) • 1:M relationship • M:N relationships • 1:1 relationship Minimum Cardinality (the “inside” symbols) • 0  optional • |  mandatory

  4. The 1:1 Relationship • One entity can be related to only one other entity, and vice versa • Could indicate that two entities actually belong in the same table • Sometimes 1:1 relationships are appropriate Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel

  5. The 1:1 Relationship Between PROFESSOR and DEPARTMENT

  6. The Implemented 1:1 Relationship Between PROFESSOR and DEPARTMENT

  7. The 1:M Relationship • Indicates that for every row in one table, there may be multiple related records • Depends on the situation • Customer to Address can be • 1:1 (e.g., HOA) • 1:M (e.g., Online retailer - mailing, billing, etc)

  8. The 1:M Relationship Between COURSE and CLASS

  9. The Implemented 1:M RelationshipBetween COURSE and CLASS

  10. The M:N Relationship • Implemented by using a pair of 1:M relationships • Can require creating a composite entity or bridge entity • Composite entity table must contain at least the primary keys of original tables

  11. The ERD’s M:N Relationship Between STUDENT and CLASS

  12. The M:N Relationship Between STUDENT and CLASS

  13. Changing the M:N Relationship to Two 1:M Relationships

  14. Converting the M:N Relationship into Two 1:M Relationships

  15. The Expanded Entity Relationship Model

  16. “Key” terms • An entity is something of importance to a user that needs to be represented in a database. • An entity represents one theme or topic and is represented by a table. • The table dimensions, like a matrix, consist of rows (tuples) and columns (attributes). • A table may be related to other tables (i.e., a relationship).

  17. Relationship Key Example Primary Key Relationship Foreign Key

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