HP Integrity NonStop J-Series - Manual (Page 95)

CREATING AND RUNNING A MULTIFILE APPLICATION
Associating and Linking Boxes
TREE(01 employee
02 dependents LINK empnum TO OPTIONAL emp-no)
S5044-024
employee
empnum
empname
• • •
Level 1
Dependents
dep-key
emp-no
dependent-no
• • •
Level 2
Link that Connects Boxes
Figure 5-5. Sample Tree Structure
What Is a Link?
A link is the portion of the tree structure that establishes a
logical connection between two boxes. A tree structure might
have many links that connect a box to several other boxes;
however, only two boxes are connected by any single link.
ENABLE provides a special kind of link called a LINK OPTIONAL.
When you establish a LINK OPTIONAL between two boxes, you define
the order in which the application can read or insert records for
the boxes. This order establishes a dependency between the
records associated with both. With a LINK OPTIONAL, the
application must use the join-field value of a record from one
box to read or insert a record for the other box. Access to a
record in the second box depends upon the presence of a record
that has a matching join-field value in the first.
Because this dependency is similar to the relationship that
exists between a parent and a child, one of the boxes connected
by a LINK OPTIONAL is called a parent box and the other is called
a child box. You can read or insert a record for a child box
only if you have already read or inserted a record for the parent
box.
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