9.2: Business Rules

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Business rules are obtained from users when gathering requirements. The requirements-gathering process is very important, and its results should be verified by the user before the database design is built. If the business rules are incorrect, the design will be incorrect, and ultimately the application built will not function as expected by the users.

Some examples of business rules are:

A teacher can teach many students.
A class can have a maximum of 35 students.
A course can be taught many times, but by only one instructor.
Not all teachers teach classes.

Cardinality and connectivity

Business rules are used to determine cardinality and connectivity. Cardinality describes the relationship between two data tables by expressing the minimum and maximum number of entity occurrences associated with one occurrence of a related entity. In Figure \(\PageIndex{2}\), you can see that cardinality is represented by the innermost markings on the relationship symbol. In this figure, the cardinality is 0 (zero) on the right and 1 (one) on the left.

Connectivity-and-Cardinality-300x202 — *Figure* \(\PageIndex{1}\): *Position of connectivity and cardinality on a relationship symbol, by A. Watt.*

The outermost symbol of the relationship symbol, on the other hand, represents the connectivity between the two tables. Connectivity is the relationship between two tables, e.g., one to one or one to many. The only time it is zero is when the FK can be null. When it comes to participation, there are three options to the relationship between these entities: either 0 (zero), 1 (one) or many. In Figure \(\PageIndex{2}\), for example, the connectivity is 1 (one) on the outer, left-hand side of this line and many on the outer, right-hand side.

Figure \(\PageIndex{2}\). shows the symbol that represents a one to many relationship.

Ch-9-oneToMany-1 — *Figure* \(\PageIndex{2}\):

In Figure \(\PageIndex{4}\), both inner (representing cardinality) and outer (representing connectivity) markers are shown. The left side of this symbol is read as minimum 1 and maximum 1. On the right side, it is read as: minimum 1 and maximum many.

Ch-9-oneToMany-2 — *Figure* \(\PageIndex{3}\):