A field property determines how the data it contains is stored, handled and displayed. A data type is regarded as a property of a field, and a field can have a number of properties depending on its data type.
The database developer can specify the length of text fields and numeric fields.
E.g. a video storeowner may find that no member has a surname longer than 20 characters. The developer can then set the members surname field length to be 25, allowing a margin of safety.
E.g. the daily rate of a video rental is usually between £1 and £3, so the daily rate field length is set at three digits, giving a maximum of £9.99. If it is defined as a currency data type, the number of decimal places is fixed at two.
The text format property specifies the display and printing of numbers, dates, times and text.
The format of the ‘issue date’ field in the video store database can be set to appear as, say, 1/4/99 or 01-Apr-99.
The format of a numeric field can be set to display a preset number of digits. A week number can only be in the range 1 to 52, and so can be set as an integer up to two digits long.
Particular text fields can be assigned a format property if they have a standard appearance, e.g. a postcode.
As with defining data types, defining the properties of a field is done when the database is created.