Databases Reference
In-Depth Information
So a temporal insert of P861, given the state of the target table
shown in
Figure 9.6
, can have an effective begin date which is
any clock tick not occupied by any of the three episodes. What,
then, of the effective end date? With a basic scenario temporal
insert, both the effective begin and end dates are left to take on
their default values which are, respectively, the date current
when the transaction is applied, and 12/31/9999. But just as
the effective begin date can be overridden on a transaction, so
too can the effective end date.
What this means is that when a new episode is created, it
does not need to be created as an open episode. If the user
happens to know the effective end date for an episode of an
object she is creating, she can specify this date on the temporal
insert, as long as it is at least one clock tick later than the effec-
tive begin date, and does not [intersect] a time period already
occupied by another episode of the same object.
In the scenario shown in
Figure 9.6
, in fact, a temporal insert
would have to specify an effective end date. The reason is that no
matter what the effective begin date is, an effective end date of
12/31/9999 would [
intersect
] one of the episodes already in the
table.
For example, a temporal insert could place a new episode
between the second two episodes, but given that our clock ticks
only once a month, it would be a tight fit. In fact, there are only
three possibilities. They are:
(i) P861[Feb 2013 - Mar 2013]
(ii) P861[Feb 2013 - Apr 2013]
(iii) P861[Mar 2013 - Apr 2013]
There are four ways in which Episode B could be {lengthened
forwards}, the last listed of which would {merge} it with Episode
C. They are:
(i) P861[Jan 2013 - Feb 2013]
(ii) P861[Jan 2013 - Mar 2013]
(iii) P861[Jan 2013 - Apr 2013]
(iv) P861[Jan 2013 - May 2013]
And there are also four ways in which Episode C could be
{lengthened backwards}, the first listed of which would {merge}
it with Episode B. They are:
(i) P861[Jan 2013 - May 2013]
(ii) P861[Feb 2013 - May 2013]
(iii) P861[Mar 2013 - May 2013]
(iv) P861[Apr 2013 - May 2013]
As we said earlier, no temporal insert would be valid that spe-
cified an effective time period of May 2013 or later. However, any
temporal insert would be valid that specified an effective time
