Databases Reference
In-Depth Information
FROM Policy_AV
WHERE asr_beg_dt
Now()
>
AND eff_beg_dt
Now()
>
As we have seen with our other re-presented pipeline
datasets, Pending Projections include both the assertion and
effective time period as part of the unique identifier because
both temporal dimensions are specified as ranges, and neither
as points in time.
Mirror Images of the Nine-Fold Way
As we said in Chapter 9, effective time exists within assertion
time. First, logically speaking, we make a statement about how
things are. Next, logically speaking, we make a truth claim about
that statement.
Most of our queries against bi-temporal tables will specify a
point in assertion time—most commonly Now()—and then ask
for rows asserted at that point in time that were in effect at
some point or period of effective time. For example, we might
ask for all policies that were in effect on August 23, 2008, as we
currently believe them to have been. Or we might ask for all
policies which we currently claim were in effect any time in
the first half of 2008.
Pinning down a point in assertion time, and then asking for
versions of objects claimed at that point in time to be correct,
is the general form that queries will take when posed by business
users. But we can look at bi-temporal data from the opposite
point of view as well. We can pin down a point or period in effec-
tive time, and ask for everything we ever asserted about things at
that point in time.
It would not be too misleading to call this the auditor's point
of view. From this point of view, we are interested in the history
of our claims about what is true, not in the history of what
actually happened out there in the world. Of course, we could
also ask for all future assertions about a given point in effective
time. But auditors, by the nature of their work, have little interest
in future assertions. By the same token, they are very interested
in past assertions, along with current ones. So an auditor's mirror-
image of the nine categories reduces to a set of six categories,
those shown in
Figure 13.13
.
These views that auditors are interested in are physically the
same ones we have already described. The “mirror-image” is in
perspective, not in content.
