Describe a new relationship between any two segments that
have at least one field in common by joining them. The underlying
data structures remain physically separate, but are treated as if
they were part of a single structure from which you can report.
This section describes how to define a join in a Master File for
FOCUS, fixed-format sequential, and VSAM data sources. For information
about whether you can define a join in a Master File to be used
with other types of data sources, see the appropriate data adapter