Databases Reference
In-Depth Information
other does not have, as well as having at least one clock tick that
the other does have.
If two time periods [
exclude
] one another, then they do not
share any clock ticks, and they are either non-contiguous or con-
tiguous. If there is at least one clock tick between them, they are
non-contiguous and we say that one is [before] the other. Other-
wise they are contiguous and we say that one [meets] the other.
If one time period [fills] the other, then either they are [equal],
or one [occupies] the other. If they are [equal], then neither has a
clock tick that the other does not have. If one [occupies] the
other, then all the clock ticks in the occupying time period are
also in the occupied time period, but not vice versa.
If one time period [occupies] the other, then either they share
an [aligns] relationship, or one occurs [during] the other. If they
are aligned, then they either start on the same clock tick or end
on the same clock tick, and we say that one either [starts] or
[finishes] the other. Otherwise, one occurs [during] the other,
beginning after the other and ending before it. Note that if two
time periods are aligned, one cannot both [start] and [finish]
the other because if it did, it would be [equal] to the other.
If one time period [starts] another, they both begin on the
same clock tick. If one [finishes] the other, they both end on
the same clock tick. If one time period [
occupies
] another, but
they are not aligned, then one occurs [during] the other.
Now let's consider the special case in which one of the two
time periods is a point in time, i.e. is exactly one clock tick in
length, and the other one contains two or more clock ticks. This
point in time may either [
intersect
] or [
exclude
] the time period.
If the point in time [intersects] the time period, it also [fills] and
[occupies] that time period. If it [aligns] with the time period,
then it either [starts] the time period or [finishes] it. Otherwise,
the point in time occurs [during] the time period. If the point
in time [excludes] the time period, then either may be [before]
the other, or they may [meet].
Finally, let's consider one more special case, that in which both
the time periods are points in time. Those two points in time may
be [equal], or one may be [before] the other, or they may [meet].
There are no other Allen relationships possible for them.
As we will see later, four of these Allen relationship categories
are especially important. They will be discussed in later
chapters, but we choose to mention them here.
(i) The [
intersects
] relationship is important because for a tem-
poral insert transaction to be valid, its effective time period
cannot intersect that of any episode for the same object
which is already in the target table. By the same token, for
