Describing database files

Records in database files can be described in two ways:

• Field level description. The fields in the record are described to the system. Some of the things you can describe for each field include: name, length, data type, validity checks, and text description. Database files that are created with field level descriptions are referred to as externally described files.
• Record level description. Only the length of the record in the file is described to the system. The system does not know about fields in the file. These database files are referred to as program-described files.

Regardless of whether a file is described to the field or record level, you must describe and create the file before you can compile a program that uses that file. That is, the file must exist on the system before you use it.

Programs can use file descriptions in two ways:

• The program uses the field-level descriptions that are part of the file. Because the field descriptions are external to the program itself, the term, externally described data, is used.
• The program uses fields that are described in the program itself; therefore, the data is called program-described data. Fields in files that are only described to the record level must be described in the program using the file.

Programs can use either externally described or program-described files. However, if you choose to describe a file to the field level, the system can do more for you. For example, when you compile your programs, the system can extract information from an externally described file and automatically include field information in your programs. Therefore, you do not have to code the field information in each program that uses the file.

The following figure shows the typical relationships between files and programs on the AS/400 system:

(1)

The program uses the field level description of a file that is defined to the system. At compilation time, the language compiler copies the external description of the file into the program.

(2)

The program uses a file that is described to the field level to the system, but it does not use the actual field descriptions. At compilation time, the language compiler does not copy the external description of the file into the program. The fields in the file are described in the program. In this case, the field attributes (for example, field length) used in the program must be the same as the field attributes in the external description.

(3)

The program uses a file that is described only to the record level to the system. The fields in the file must be described in the program.

Externally described files can also be described in a program. You might want to use this method for compatibility with previous systems. For example, you want to run programs on the AS/400 system that originally came from a traditional file system. Those programs use program-described data, and the file itself is only described to the record level. At a later time, you describe the file to the field level (externally described file) to use more of the database functions available on the system. Your old programs, containing program-described data, can continue to use the externally described file while new programs use the field-level descriptions that are part of the file. Over time, you can change one or more of your old programs to use the field level descriptions.