CREATING THE LOGICAL MODEL - A Developer's Guide to Data Modeling for SQL Server - page 163

Table 7.2

A Sample of the Relationship List for Mountain View Music

Parent Entity Child Entity Type Cardinality

Bank Accounts None N/A N/A

Bins Product Instances M, I One to zero or more

Credit Cards None N/A N/A

Customers Orders M One to zero or more

Shopping Cart M, I One to zero or more

Employees Orders M One to zero or more

Purchases M One to zero or more

Gift Cards None N/A N/A

Payments Bank Accounts S Exclusive

Credit Cards S Exclusive

Gift Cards S Exclusive

Type: M = Mandatory, I=Identifying, S=Subtype

Remember that this is a short list of relationships. The total list will be
large, because there will be an entry in the Parent Entity column for every
entity in the model. This comprehensive list serves as a single source of
information as you work through building your model in the modeling
software.

Business Rules
Business rules, as discussed in Chapter 6, can be implemented in various
ways throughout an IT system. Not all business rules will be implemented
in the data model and ultimately the physical database. Because we're not
inviting debate on exactly where all business rules should go, we focus on
those that belong in the data model, usually because they specifically relate
to data integrity.

Types of Rules Implemented in a Logical Model
In general, all the relationships that dictate whether or not data can be
added, updated, or deleted from a database are types of business rules. For
example, if a company requires that a valid phone number be stored for a
customer—whether it is a cell phone, a home phone, or a work phone—
you can create a constraint to prevent the customer record from being
saved without at least one of those fields containing data.