Hermit's Free Library Database Design Cases
Hermit's Free Library Database Design Cases
This is the second in a series of database design case studies. This database consists of three tables. It is recommended to begin with the first case study, which illustrates a two-table database.
Design a simple relational database which would track Social Security contributions for workers.
The database must provide the following information:
The first step in database design is to determine all the necessary fields and segregate them into groups that mirror the natural order of the real world. In this case we have Workers, Employers, and Contributions.
When the fields have been grouped into their natural order, each group represents a table.
Each record in a table must be uniquely identified by its value in the table's key field.
The next step is to identify the relationships between the tables. In this case we have a many-to-many relationship between Workers and Employers. That is to say that Workers may have many employers over their working lives, and Employers can have many Workers.
When two tables have a many-to-many relationship we create a third table, often referred to as a lien table because it completes the many-to-many relationship.
Notice that the relationship between both many-to-many tables and the lien is a one-to-many relationship.
As with all one-to-many relationships, we place values from the "one" table's key field into a field of the related records in the "many" table. So in this example the primary keys from both "one" tables (Workers and Employers) are secreted into the "many" table (Contributions) as foreign keys.
The final result is a relational database model. When a database model satisfies all the technical requirements it is said to be rationalized.
The objective of rationalizing a database structure is to eliminate redundancy in order to save disk space and to simplify updates by insuring that every piece of information exists in only one place. The only fields that repeat are the foreign keys.
Every rational database model must obey these four rules and pass these three tests.
