What Does a Master File Describe?

In this section:

Identifying a Data Source
Identifying and Relating a Group of Fields
Describing a Field

A Master File enables you to:

Identify the name and type of a data source.
Identify and relate groups of fields.
Describe individual fields.

Note:

Every Master File must contain at least one segment declaration and one field declaration. If a required attribute is not assigned a specific value, a comma must be used as a placeholder.
The maximum number of segments supported in a structure is 1024. This structure can be created by joining or combining data sources.
The maximum number of segments in a single FOCUS data source is 64. XFOCUS data sources can have up to 512 segments. The maximum number of segments plus indices for a single FOCUS data source is 191.
The total length of all fields can be up to 256K.

Identifying a Data Source

In order to interpret data, your application needs to know the name you are using to identify the data source and what type of data source it is. For example, is it a DB2 data source, an Oracle data source, or a FOCUS data source?

For more information, see Identifying a Data Source.

Identifying and Relating a Group of Fields

A Master File identifies and relates groups of fields that have a one-to-one correspondence with each other. In Master File terms, a segment and in relational terms, a table.

You can join data sources of the same type (using a Master File or a JOIN command) and data sources of different types (using a JOIN command). For example, you can join two DB2 data sources to a FOCUS data source, and then to a VSAM data source.

For more information about defining and relating groups of fields, see Describing a Group of Fields.

Describing a Field

Every field has several characteristics that you must describe in a Master File, such as type of data and length or scale. A Master File can also indicate optional field characteristics. For example, a Master File can specify if the field can have a missing value, and can provide descriptive information for the field.

A Master File usually describes all of the fields in a data source. In some cases, however, you can create a logical view of the data source in which only a subset of the fields is available, and then describe only those fields in your Master File.

For more information, see Describing an Individual Field.

Note: Master Files/data source descriptions must contain uppercase field and segment names if you are using them with Maintain Data.