Appendix Clinks And The Link Optional Statement Rules; How Enform Defines A Link; Links Initiated By A Link Statement; Links Initiated By A Link Optional Statement - Tandem ENFORM 058057 Reference Manual

Appendix C
How ENFORM Defines
a LINK
Links Initiated By A LINK
Statement
Links Initiated by a LINK
OPTIONAL Statement
LINKS and the LINK OPTIONAL
Statement Rules
To fully understand the rules for the LINK OPTIONAL statement, you must first
understand how ENFORM defines links. This appendix describes how ENFORM
defines a link, reviews the rules for the LINK OPTIONAL statement, and provides
some examples of illegal links.
ENFORM recognizes links that are established:
1. by a LINK statement that references both record descriptions
2. by a LINK OPTIONAL statement that references both record descriptions
3. by a WHERE clause containing a term that references both record descriptions
4. through the transitive property of links.
The following paragraphs discuss the characteristics of these links.
A link initiated by a LINK statement is a two-directional link. This means that a two-
way dependency condition exists between the record occurrences associated with the
linked record descriptions. For example, if you specify:
LINK apple TO orange VIA seeds;
the following is true of the logical records built for the link:
A given apple record occurrence appears in the logical records only if its linking
field value matches the linking field value in an orange record occurrence. Thus,
an apple record occurrence depends upon an orange record occurrence.
A given orange record occurrence appears in the logical records only if its linking
field value matches the linking field value in an apple record occurrence. Thus, an
orange record occurrence depends upon an apple record occurrence.
A two-directional link established for a LINK statement can be sketched as follows:
record-description-1
where the two-headed arrow indicates the two-way dependency condition.
A link initiated by a LINK OPTIONAL statement is a one-directional link. This means
that a one-way dependency condition exists where the record occurrences associated
with the record description on the right of the statement depend upon the record
occurrences associated with the record description on the left of the statement. For
example, if you specify:
LINK apple TO OPTIONAL orange VIA seeds;
058057 Tandem Computers Incorporated
record-description-2
C–1