Draw an ER diagram for the three tables: Customer, Employee, and Sales Order.
Suppose the Sales Order Table also contains information for an employee who makes a sale. If you have an ERD drawing tool such as Visio, you could draw the diagram using the tool and copy it into your document.
You could also draw it on paper, scan it, and insert it into your document. If you can't do either of the above, you could write down the table schemata and describe the relationships among them.
Databases often hold a great amount of data. To build a database, we need to understand which entities should hold data, and identify the connections that may exist between entities.
This module discusses the entity relationship model, which will allow us to create a graphical view of the different elements of a database as well as the relationships between them. We will also learn the drawing conventions of the ER model using a part-to-whole approach, beginning with conventions used to represent a single entity, and concluding with conventions used to represent all relations in a database.
Data Modeling is the process of creating a data model for a given set of data, and it is a major component of software development. The way data is modeled is very important in how it can be accessed and manipulated using SQL.
The dominant design methodology for relational databases consists of three steps:
1. Identify all relevant entities and relationships, and describe them using ER diagrams.
2. Convert the ER model to a number of relation schemas.
3. Eliminate (or reduce) redundancy by splitting relations. This process is called normalization.
In most cases, an entity can be identified uniquely in its entity set by the values of its attributes. A relationship in the ER model is exactly the relation in the relational data model.
- Entity sets correspond to the domains of the values.
- Tuples correspond to connections between entities.
- Relationship instances form a table, like relation instances.
In the Books example from Module 1, books and authors are two entity sets. Each tuple (book, author) corresponds to a connection between two specific entities in the sets. Books and their author relationships form the table below.
|
Books
|
Authors
|
|
The Lost Symbol
|
Dan Brown
|
|
Superfreakonomics
|
Steven Levitt and Stephen Dubner
|
|
Lego Star Wars
|
Simon Beecroft
|
|
...
|
...
|
Good data modeling can be difficult because there may be many data models to choose from. The following design principles can be useful when considering a design:
- Be faithful to the specification of the application.
- Avoid duplication and other redundant information.
- The KISS principle (Keep It Simple, Stupid).
- Choose the right relationship.
- Use attributes when possible.
Study the required materials, and make sure you understand the core concepts introduced there, mainly the entity relationship diagram and its components: entities, attributes, and relationships.
Attachment:- Required Reading.rar