Introduction to Logical Design › Determining the Users' Data Needs

Determining the Users' Data Needs

Users of application programs require access to only selected portions of a database. Therefore, you need to develop a logical model that includes descriptions of the data required by each program.

Data tables

To the user of an application program, information in a CA IDMS/DB database will appear in the form of data tables. Data tables consist of columns and rows of related data. For example, a table might contain information about a company's departments, organized under headings such as DEPT ID, DEPT NAME, and DEPT BUDGET. A DEPARTMENT table with these categories of information is illustrated in the following diagram.

Information for company departments is maintained in the Department data table. A column represents a list of all department IDs. A row represents a single department.

Views

Users can manipulate columns and rows of data by accessing tables directly or by defining views of the database. Views enable users to select specified rows or columns or to combine information from two or more tables. For example, a view might use the relational join operation to combine information from the DEPARTMENT table and the EMPLOYEE table, as illustrated below.

Relational join operation

To show company employees with their departments, the DEPT/EMPLOYEE view uses the common DEPT ID column to join the Department and Employee data tables. This join operation selects all information from the tables that pertains to department 110. In the DEPT/EMPLOYEE view, the project operation has been used to include the DEPT ID and DEPT NAME columns from the DEPARTMENT table and the EMP # and LAST NAME columns from the EMPLOYEE table.