These topics introduce several methods for describing iSeries™ database files, with focus on how to describe database files using data description specifications (DDS) because DDS has the most options for defining data.
If you want to describe a file just to the record level, you can use the record length (RCDLEN) parameter on the Create Physical File (CRTPF) and Create Source Physical File (CRTSRCPF) commands. If you want to describe your file to the field level, several methods can be used to describe data to the database system: interactive data definition utility (IDDU), Structured Query Language (SQL) commands, or data description specifications (DDS).
Physical files can be described using IDDU. You might use IDDU because it is a menu-driven, interactive method of describing data. You might be familiar with describing data using IDDU on a System/36™. In addition, IDDU allows you to describe multiple-format physical files for use with Query, iSeries Access, and data file utility (DFU).
When you use IDDU to describe your files, the file definition becomes part of the i5/OS™ data dictionary.
SQL can be used to describe an iSeries database file. It supports statements to describe the fields in the database file, and to create the file.
SQL was created by IBM® to meet the need for a standard and common database language. It is currently used on all IBM DB2 platforms and on many other database implementations from many different manufacturers.
When database files are created using the DB2 UDB for iSeries SQL language, the description of the file is automatically added to a data dictionary in the SQL collection. The data dictionary (or catalog) is then automatically maintained by the system.
SQL is the language of choice for accessing databases on many other platforms. It is the only language for distributed database and heterogeneous systems.
Externally described data files can be described using DDS. Using DDS, you provide descriptions of the field, record, and file level information.
You might use DDS because it provides the most options for the programmer to describe data in the database. For example, only with DDS can you describe key fields in logical files.
The DDS form provides a common format for describing data externally. DDS data is column sensitive. The examples in this manual have numbered columns and show the data in the correct columns.
After a database file is described, you can view the description.