Describing database files using DDS

When you describe a database file using data description specifications (DDS), you can describe the file at the file, record-format, join, field, key-field, and select/omit-field levels.

File-level DDS provides the system information about the entire file. For example, you can specify whether all the key field values in the file must be unique.
Record format-level DDS provides the system information about a specific record format in the file. For example, when you describe a logical file record format, you can specify the physical file that it is based on.
Join-level DDS provides the system information about the physical files that are used in a join logical file. For example, you can specify how to join two physical files.
Field-level DDS provides the system information about individual fields in the record format. For example, you can specify the name and attributes of each field.
Key field-level DDS provides the system information about the key fields for the file. For example, you can specify which fields in the record format are to be used as key fields.
Select/omit field-level DDS provides the system information about which records are to be returned to the program during file processing. Select/omit specifications apply to logical files only.