Databases Reference
In-Depth Information
Key of a table cascades to any other table that references that table, causing
an increment in the Instance Key of the referring table. A table at the top of a
hierarchy structure will not experience the effect of cascading Instance Keys
because a table at the top of a hierarchy structure refers to no hierarchical
parents. A table at the bottom of a hierarchy structure, however, will experi-
ence significant effects from cascading Instance Keys because a table at the
bottom of a hierarchy references hierarchical parents that also have hierar-
chical parents (and probably grandparents) of their own. A table, such as the
Item table in Figure 11.4 and Figure 11.5, that refers to multiple hierarchical
parents will experience cascading Instance Keys from both hierarchical par-
ent tables.
This is the point at which this Time Variant Solution Design initially
seems somewhat counterintuitive. The insertion of additional rows in a
hierarchically lower table because a h ierarchically higher table experi-
enced updates seems to defy common sense. But then, a good example
of why this construct succeeds can be found in the automotive insurance
industry. For an example of the automotive insurance industry, we can
consider a fictitious young boy named Harold.
• Harold/16 years of age—At the age of 16 years, Harold receives
his driver's license. His automotive insurance company requires a
very high insurance premium because Harold is 16 years of age; all
16-year-old boys are considered a very high risk.
• Harold/24 years of age—At the age of 24 years, Harold graduates
from college. His automotive insurance company reduces his insur-
ance premium because Harold is 24 year of age and has a college
degree; all 24-year-old young men with a college degree are consid-
ered a moderate risk.
• Harold/35 years of age—At the age of 35 years, Harold is married
and has his first child. His automotive insurance company extends
its lowest possible insurance premium because Harold is 35 years of
age and has a child; all 35-year-old men with a child are considered
a minimal risk.
Is there a difference between Harold/16, Harold/24, and Harold/35? Yes,
there is a d ifference. The difference is that Harold/16 is a member of a
high-risk group, whereas Harold/24 is a member of a moderate-risk group
and Harold/35 is a member of a low-risk group. Are Harold/16, Harold/24,
and Harold/35 the same person? Yes, of course they are all Harold.
