BTEC National Extended Diploma1 IT Systems Analysis and Design Entity Relationship Diagrams

BTEC National Extended Diploma2 Aims Gain some understanding into Entity Relationship Diagrams (ERD) and what they are used for. Attempt to understand the decomposed process Attempt to create an ERD diagram

BTEC National Extended Diploma3 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 Extended Diploma4 Entity Relationship Diagrams ERD – Entities  Real world things that are part of the system under investigation.  Examples include Products, Customers, Orders.

BTEC National Extended Diploma5 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 Extended Diploma6 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 Extended Diploma7 Entity Relationship Diagrams OrderIDEmployeeIDProductIDOrderDateDeliver_byComments 1100A129/03/200929/04/2009Ring the bell 1100A229/03/200929/04/2009Ring the bell 1100A329/03/200929/04/2009Ring the bell 2101A116/04/200917/04/2009N/A 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 Extended Diploma8 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 Customer Can Make M 1

BTEC National Extended Diploma9 Entity Relationship Diagrams OrderI D Product ID Qty 1A11 1A215 1A323 2A112 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. OrderIDEmployeeIDCustIDOrderDateDeliver_byComments 1100C129/03/200929/04/2009Ring the bell 2101C216/04/200917/04/2009N/A Order table EmployeeIDEmployee Name 100John Smith 101Mia Lee Product Employee ProductIDProductName A1Football A2Tennis Ball A3Golf Clubs CustIDCust Name C1Gianni Albin C2Martin Jones Customer

BTEC National Extended Diploma10 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 /03/200929/04/2009Ring the bell These are all attributes, they have a name and a value.

BTEC National Extended Diploma11 Conclusion Gained some understanding into Entity Relationship Diagrams (ERD) and what they are used for. Attempted to understand the decomposed process Attempted to create an ERD diagram