A relational database is expressed logically by tables.

The values in the vertical and horizontal directions of a table are called columns and rows, respectively. The values within a column represent data with the same attribute, that is, the same data type. A table consists of a set of rows; the row is the basic unit for retrievals. Each column is assigned a name (column name) that is used for database manipulations.

Figure 2-1 shows an example of a basic table configuration. Ending zeros in the PRICE column (in this example and throughout the manual) are not displayed on the actual screen.

Figure 2-1 Basic table configuration example

A repetition column refers to a column that consists of multiple elements. Using repetition columns has the following advantages:

• Multiple tables do not have to be joined.
• Less disk space is used because many duplicate information items are eliminated.
• Access performance is better because related data items (repetition data items) are stored near each other rather than in separate tables.

Figure 2-2 shows a configuration example of a table that has repetition columns.

Figure 2-2 Configuration example of a table with repetition columns

A virtual table that limits the range of columns or rows that can be manipulated by the user can be created based on an actual table (referred to hereafter as a base table). Such a virtual table is called a view table. A view table can be defined for the following purposes, thus restricting the manipulation range and simplifying operations:

• To retrieve only certain columns of a table
• To change the order of the columns in a table
• To retrieve only certain rows of a table

Although a view table is usually defined to view only selected columns or rows of a table, it can be retrieved in the same way as a base table. Because use of a view table restricts the range of manipulations that are possible, precise security measures can be implemented by means of view tables.

Figure 2-3 shows an example of a view table created from a base table.

For details about how to define and manipulate a view table, see 2.11 Defining and manipulating a view table.

Figure 2-3 Example of a base table and view table