Databases Reference
In-Depth Information
is a parent to the xref table, which is thus RI dependent on both
of them.
Each row in the xref table has two FKs, one to a parent row in
one table and one to a parent row in another table (or, possibly,
in the same table). As we already know, this dual RI dependency
means that a row cannot be inserted into the xref table unless
both its parent rows already exist in the database, and neither
parent row can be deleted as long as that xref row remains in
the database.
TRI with Multiple TFKs
If a child version has two or more TFKs, the effective
timespan of an episode of each of the objects which those TFKs
reference must fully include the effective timespan of the ver-
sion. If either of them did not, that would be a TRI violation.
So consider an associative asserted version table, whose vers-
ions each contain two TFKs. What of the Allen relationships
between the two parent episodes related by any version in this
table? Are there any constraints on those parent episodes?
In fact, there are. Those two effective timespans must [
inter-
sect
]. If they did not [
intersect
], then there would be no clock
tick when both were in effect, and so no clock tick in which an
xref row, TRI dependent on both parents, could exist.
Consider an example in which we have a customer episode
C773-B with an effective timespan from March 2013 until further
notice, which we will write as C773-B[Mar 2013 - 12/31/9999],
and also a salesperson episode S217-D[Sep 2013 - Dec 2013].
What can we say of the effective timespan of a version in an
asserted version associative table relating that customer episode
to that salesperson episode?
7
First, that associative table version cannot have an effective
begin date prior to September 2013 because that would make
the start of its effective time period earlier than the start of
S217-D. By the same token, that version cannot have an effective
end date after December 2013 because that would make the end
of its effective time period later than the end of S217-D.
So knowing what we do of the two parent episodes, what is
the maximum effective timespan that would be valid for the
7
As a complete aside, we note that the in-line notations developed in Chapter 6 and
elsewhere in this topic, for example the S217-D[Sep 2013 - Dec 2013] notation
developed in this chapter, might be the basis for a degree of automated semantic
interoperability between structured and semi-structured representations of temporal
data.
