Technical data

Figure 5–13 Forked Set Occurrence
PART_INFO
ZK−1491−GE
PART
SUPPLY
PR_QUOTE
SUPPLY PR_QUOTE
PR_QUOTE
5.9.2 Multiset Relationships
A set cannot contain an owner record and a member record of the same type.
Nor can a simple set represent a many-to-many relationship. To simulate such
relationships, Oracle CODASYL DBMS uses the concept of multiset relationships.
Multiset relationships occur when two set types share a common record type
called a junction record. The junction record can contain information specific
to the relationship. An empty junction record contains only pointer information
used by the DBCS to establish the multiset relationship. This section discusses
three kinds of multiset relationships:
• Many-to-many relationships between two types of records
• Many-to-many relationships between records of the same type
• One-to-many relationships between records of the same type
5.9.2.1 Many-to-Many Relationships Between Two Types of Records
To build a many-to-many relationship between two types of records, the DBA uses
a junction record. For example, a part can be supplied by many vendors, and one
vendor can supply many parts. The SUPPLY record type in Figure 5–14 links or
joins PART records with VENDOR records.
Database Programming with HP COBOL 5–15