Btec National - Advanced Databases 1 Advanced Databases Entity Relationship Diagrams

Btec National - Advanced Databases 2 Aims Learn about Entity Relationship Diagrams (ERD) and what they are used for. Understand the decomposed process Create an ERD diagram

Btec National - Advanced Databases 3 Entity Relationship Diagrams ERD –An ERD represents the entities and the relationships that exists between them. –An ERD diagram is created to establish what tables within a database are required and what foreign keys are needed

Btec National - Advanced Databases 4 Entity Relationship Diagrams ERD – Entities –Real world things that are part of the system under investigation. –Examples include Products, Customers, Orders.

Btec National - Advanced Databases 5 Entity Relationship Diagrams ERD – Relationship –Relationships illustrate how two entities share information in the database structure. –Relationships exists between each of the entities examples are shown below: 1M MM 11

Btec National - Advanced Databases 6 Entity Relationship Diagrams ProductsEmployeeOrder MM M1 Makesinvolves A many to many relationship as shown above is not allowed because it will create duplicate data, and repeating groups. For more information use the link below: eating_groups.htm

Btec National - Advanced Databases 7 Entity Relationship Diagrams OrderIDEmployeeIDProductIDOrderDateDeliverbyComments 1100A129/03/200929/04/2009 Ring the bell 1100A229/03/200929/04/ A329/03/200929/04/2009 Order table This is the result of a many to many relationship, as you can see there is repeating groups of data (A1, A2,A3), the OrderID primary key has lost it’s uniqueness and there is loads of redundancy i.e. the order date 29/03/2009 is stored more than once.

Btec National - Advanced Databases 8 Entity Relationship Diagrams Products EmployeeOrder M1 Makes To resolve this many to many relationship we have to add another table which is a decomposed table of order and products, called Order Products. Order Products M1 Consists M Included 1

Btec National - Advanced Databases 9 Entity Relationship Diagrams OrderIDProductID 1A1 1A2 1A3 Order Products By using the decomposed table it reduced redundancy and removed repeating groups from the Order table, this results in a better functioning database.OrderIDEmployeeIDOrderDateDeliverbyComments110029/03/200929/04/2009 Ring the bell Order table

Btec National - Advanced Databases 10 Entity Relationship Diagrams ERD – Attribute –An attribute is a specification that defines a property of an entity. An attribute of an entity usually consists of a name and a value for example: OrderIDEmployeeIDOrderDateDeliverbyComments /03/200929/04/2009 Ring the bell These are all attributes, they have a name and a value.

Btec National - Advanced Databases 11 Conclusion Learnt about Entity Relationship Diagrams (ERD) and what they are used for. Discussed the decomposed process Created an ERD diagram