Database Management
Reference Manual
Introduction to Database Concepts
:
Database Expressions and Rules
: Rules
Rules
An attribute may be defined as a rule. For example, the attribute XLEN may be defined as a rule by the expression (YLEN * 2).
The OF syntax is often used in Rule expressions to refer to other elements, e.g. (YLEN OF /FRED * 2)
The result of the rule is stored against the attribute as for other attributes.
There are commands to recalculate the rule.
Rules may be either static or dynamic. If static, then the rule result will only be recalculated on demand. If dynamic, then the result will be recalculated every time an attribute within the expression changes, e.g. for the above rule, if YLEN is modified, then XLEN will be recalculated automatically. The dynamic linkage of rules may be across elements and across DBs.
1974 to current year.
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