Define business rules for a company that sells t-shirts online.
Define at least 6 business rules with respect to entity classes and their relationships for this business.
There are many customers buying many different t-shirts in many different sizes and styles.
Then draw an E-R diagram based on the 6 or more rules defined.
What you're being asked to do is to define business rules and develop entity relationships for a hypothetical business with broad, though not deep, parameters of operation through hyperspace over the Internet (the store doesn't have a brick-and-mortar shop space) and with a need for database development to map and organize those entity relationships.
Define Business Rules
First you must define the general concept of business rules then identify the specific ones applicable to database development of entity relationships for the T-shirt business selling many different kinds of T-shirts, each available in many sizes and many styles (which is not redundant of "different T-shirts" though it may seem to be).
Business rules are defined, briefly, as some statement that defines (allows) or constrains (prohibits) some aspect of operation. The classic example of this as used in the Business Rules Project is the constraining rule that a customer order will not be accepted if the customer credit rating is below a specific credit score. The rule may also be expressed in the reverse as an defining rule by stating that customer orders will be accepted if their credit rating is above a specific credit score. Business rules define (allow) or constrain (prohibit) aspects of business operation.
A present real-life example of a business rule comes from OnlineShoes.com. They recently changed a business rule to allow for installment payments on online shoe purchases. Their new constraining business rule might be expressed approximately like this: Customer orders will be accepted as 3, 6 or 12 month installment payments if the purchase is made on a valid credit card as set for automatic monthly recurring ACH/EFT transactions.
Business Rules Categories
Business rules are most efficient and effective when written and encoded in a database. The widely accepted database for encoding business rules is the entity-relationship diagram, which creates a visualization of entities integral to a business, the relationships between them and business rules governing them. There are four categories of business rules governing structure, function, and defining and constraining conditions.
- Business terms: briefly, the definition of entities.
- Facts relating terms: briefly, the description of relationships between entities.
- Constraints on behavior: briefly, what is disallowed in business operation and in data entry/update.
- Derivation of facts: briefly, derived facts based on inference or on mathematical calculation that modify knowledge so it both accords with and in used in application of business rules.
These four categories of business rules will be applicable to the online T-shirt business:
- entities must be identified and defined;
- relationships must be defined and traced;
- constraints must be established;
- facts/knowledge of Internet enterprise must be derived (modified) to fit the business rules of the T-shirt business.
Entity-Relationship Diagram (ERD)
Entities are the objects and people that are integral to the operation of business. Examples of entities are the owner and order processors, the accountant, the place of business or shipping, the product and activity types, and the individual products and activities, like T-shirts and order fulfillment.
Relationships exist between entities. For example, a relationship exist between the two entities defined as the customer placing the order and the employee fulfilling the order to be shipped. Entity-relationship diagrams visually identify these entities and visually establish the relationships between them.
The T-shirt company has some easily identifiable entities: the proprietor, each customer who initiates an account (with elements of individual accounts comprising subtypes of entities), each type of T-shirt (with sizes and styles comprising subtypes of entities), order takers, order fulfillers, order shippers and the activities of order processing, fulfilling and shipping.
Each entity will be defined in terms of its attributes (or characteristics, like account holder password and address, for example) and the relationships stemming from these attributes. Each attribute becomes a subtype of entity and comprises part of the relationship. One image attached develops an example of an ERD based upon Peter Chen's groundbreaking work. The other develops an ERD for a single account holder participating at an online game site.