1. Microsoft Access CS 110 Fall 2005

2. Entity Relationship Model Entities Entities Principal data object about which information is to be collectedPrincipal data object about which information is to be collected employees, projects, invoices, etc. employees, projects, invoices, etc. Relationships Relationships An association between two or more entitiesAn association between two or more entities employees are assigned to projects employees are assigned to projects

3. Entities Entities have attributes Entities have attributes name, ID, keyname, ID, key May be a way to uniquely identify an instance of an entity May be a way to uniquely identify an instance of an entity May be a non-unique characteristic of an entity instance May be a non-unique characteristic of an entity instance

4. Relationships Relationships have connectivity and cardinality Relationships have connectivity and cardinality One-to-one – at most one instance of entity A is associated with one instance of entity BOne-to-one – at most one instance of entity A is associated with one instance of entity B One-to-many – one instance of entity A is associated with zero, one, or many instances of entity BOne-to-many – one instance of entity A is associated with zero, one, or many instances of entity B Many-to-many – one instance of entity A is associated with zero, one, or many instances of B (and vice versa)Many-to-many – one instance of entity A is associated with zero, one, or many instances of B (and vice versa)

5. Building a data model Identify entities and relationships Identify entities and relationships What are the entity attributes What are the entity attributes Confirm the desired query results can be generated Confirm the desired query results can be generated

6. What makes an object an entity or an attribute? “Employees work on projects” “Employees work on projects” Are employees an entity or an attribute of projects? Analyze narratives from users, meeting notes, policy and procedure documents, … Analyze narratives from users, meeting notes, policy and procedure documents, … Consider database properties Consider database properties

7. Normalization Put data into table form by removing repeated groups and duplicated data Put data into table form by removing repeated groups and duplicated data A theoretical definition based on logic existsA theoretical definition based on logic exists

8. Example A company obtains parts from multiple suppliers A company obtains parts from multiple suppliers Each supplier is in a city Each supplier is in a city A city can have more than one supplier A city can have more than one supplier Each city has a status code Each city has a status code Each supplier may provide many parts Each supplier may provide many parts

9. Example s#: supplier ID s#: supplier ID status: city status status: city status city: name city: name p#: part ID p#: part ID qty: quantity qty: quantity

10. Redundant data City and status repeated for each part City and status repeated for each part

11. Update anomalies Problems that occur when information is inserted, deleted, or updated Problems that occur when information is inserted, deleted, or updated Caused by redundant data Caused by redundant data

12. Insert anomalies Cannot add new supplier w/o adding part supplied Cannot add new supplier w/o adding part supplied

13. Delete anomalies Deleting a row loses information about quantity/part AND supplier Deleting a row loses information about quantity/part AND supplier

14. Update anomalies If supplier s1 moves from NY to London, six rows must be updated If supplier s1 moves from NY to London, six rows must be updated

15. Refinement Following rigor of normalization, tables are split/merged to reduce anomalies Following rigor of normalization, tables are split/merged to reduce anomalies

16. Primary, secondary, and foreign keys Primary key: attribute that uniquely identifies all other attributes in a row Primary key: attribute that uniquely identifies all other attributes in a row Secondary key: attribute that identifies a set of records Secondary key: attribute that identifies a set of records Foreign key: attribute in one table that matches the primary key of another Foreign key: attribute in one table that matches the primary key of another