Logical and Physical Data Structures

When data structures are first defined, only the data elements that the user would see, such as a name, address, and balance due, are included. This stage is the logical design, showing what data the business needs for its day-to-day operations. As we learned from HCI, it is important that the logical design accurately reflect the mental model of how the user views the system. Using the logical design as a basis, the analyst then designs the physical data structures, which include additional elements necessary for implementing the system. Examples of physical design elements are the following:

1. Key fields used to locate records in a database table. An example is an item number, which is not required for a business to function but is necessary for identifying and locating computer records.
2. Codes to identify the status of master records, such as whether an employee is active (currently employed) or inactive. Such codes can be maintained on files that produce tax information.
3. Transaction codes are used to identify types of records when a file contains different record types. An example is a credit file containing records for returned items as well as records of payments.
4. Repeating group entries containing a count of how many items are in the group.
5. Limits on the number of items in a repeated group.
6. A password used by a customer accessing a secure Web site.

Figure illustrated below is an example of the data structure for a CUSTOMER BILLING STATEMENT, one showing that the ORDER LINE is both a repeating item and a structural record. The ORDER LINE limits are from 1 to 5, indicating that the customer may order from one to five items on this screen. Additional items would appear on subsequent orders.

The repeating group notation may have several other formats. If the group repeats a fixed number of times, that number is placed next to the opening brace, as in 12 {Monthly Sales}, where there are always 12 months in the year. If no number is indicated, the group repeats indefinitely. An example is a table containing an indefinite number of records, such as Customer Master Table  {Customer Records}.

The number of entries in repeating groups may also depend on a condition, such as an entry on the Customer Master Record for each item ordered. This condition could be stored in the data dictionary as {Items Purchased} 5, where 5 is the number of items.