Databases Reference
In-Depth Information
semantics. One exception is created by the presence of deferred
assertions in the table which are, by definition, assertions whose
begin dates, at the time the transaction is processed, lie in the
future. The other exception is created when assertions are with-
drawn without being replaced or superceded, i.e. when after a
certain point in time we no longer wish to claim that those
assertions ever were, are or will be a true description of what
their object was, is or might be like during some stretch of effec-
tive time. But as we said earlier, we will not discuss deferred
assertions until several chapters from now, at which time we will
also discuss withdrawn assertions that are not replaced and/or
superceded by other assertions.
Each of these assertion time snapshots consists of one or
more boxes. As we said, each box contains the row number of
the row it represents. The vertical line on the left-hand side of
each box corresponds to the effective begin date of the row it
represents. In this illustration, only one of the boxes is closed,
in the sense of having a line on its right-hand side. The other
two are open both graphically and, as we will see, semantically.
Let's consider these boxes, one at a time. The box for row 1 is
open-ended. This always means that the corresponding row has
an effective end date of 12/31/9999. The box directly below the
box for row 1 represents row 2. Because that box is closed on
its right-hand side, we know that the row it represents has a
known effective end date which, in this case, is May 2010.
In these boxes that line up one under the other, the business
data in the rows may or may not be identical. If the business
data is identical, then the box underneath the other represents
a replacement, and we will indicate that it has the same business
data as the row it replaces by using the row number of the row it
replaces as a superscript. But if the business data in two rows for
the same object, during the same effective time period, is not
identical, then the row represented by the lower box supercedes
the row represented by the upper box, and in that case we
will not include a superscript. This convention is illustrated in
Figure 6.2
, in which the box for row 2 has a superscript
designating row 1, whereas the box for row 3 has no superscript.
The box directly to the right of the box for row 2 represents
row 3. We can tell that the two rows are temporally adjacent
along their effectivity timelines because the same vertical line
which ends row 2's effective time period also begins row 3's
effective time period. So this diagram shows us that there is an
unbroken effective time period for policy P861, which began to
be asserted on May 2010, and which extends from row 2's effec-
tive begin date of January 2010 to row 3's effective end date of
