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Relationships

Relationships. Today you are Learning. Understand the need for a relational database structure Be able to create ERDs of the relationship Use software to model and create a relational database. 10/03/2014. R.D.B.M.S - Entities.

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Relationships

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  1. Relationships

  2. Today you are Learning • Understand the need for a relational database structure • Be able to create ERDs of the relationship • Use software to model and create a relational database 10/03/2014

  3. R.D.B.M.S - Entities • A relational database consists of multiple tables joined (linked) together by relationships • Each table should contain data about a singleentity • Person – Employee • Object - Book • Event - Project

  4. R.D.B.M.S - Attributes • Attributes describe the properties of an entity • It’s easiest to think of attributes as fields

  5. Relationships

  6. ERDs Entity relationship diagram

  7. Relationships • One to one 1:1 • One to many 1:M • Many to Many M:M

  8. Relationships • One to one 1:1 • Teacher - classrooms • One to many 1:M • Teacher - courses • Many to Many M:M • Teachers - Students

  9. Relationship?

  10. Money

  11. Relationships.....

  12. Relationships.....

  13. Draw the relationship shown below • One to one 1:1 • Teacher - classrooms • One to many 1:M • Teacher - courses • Many to Many M:M • Teachers - Students

  14. Worked Example 1 • Look at the iTunes database on the next slide • Draw an ERD of the entities you would expect to see if this was converted into a relational databaseHINT: There are 4 entities. All relationships are one-to-many

  15. Worked Example 2

  16. Worked Example 3 • Did you get something like this? Artists Album Name Track Name Genres

  17. Creating a relationship • Load up the Actors database from the Cambridge Tech website • Create a ‘Oscars’ Table to join the data

  18. ENFORCE REFERENTIAL INTEGRITY • An enforced relationship ensures each value entered in a foreign key column matches an existing value in the related primary key column.

  19. Cascade Update Fields • cascading updates only update existing records so that if you were to make a change to the customers table, all instances of the customer id would be changed in the customer orders table.

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