Databases Reference
In-Depth Information
The [before], [before
1
], [meets] and [meets
1
] relationships
are subtypes, in our taxonomy, of the [
excludes
] relationship.
And we can now see why this is an important group of
relationships to define. Temporal insert transactions are valid
only if they have an [
excludes
] relationship with every other
episode of the same object already in the target table. And by
the same token, temporal update and temporal delete trans-
actions are valid only if there is at least one episode of the
same object already in the target table with which they do
not have an [excludes] relationship. So now that we are through
with the [excludes] branch of the Allen taxonomy, we have
exhausted all the Allen relationship possibilities for temporal
insert transactions.
We will discuss how temporal delete transactions work with
the remaining Allen relationships. We will not explicitly discuss
temporal updates because temporal updates are semantically
equivalent to temporal deletes followed by the insertion of
updated versions which supercede those versions wholly or par-
tially withdrawn. And so there are no Allen relationships possible
for temporal updates that are not also possible for temporal
deletes.
[Starts]
. If a temporal delete transaction's effective time
begins on the same clock tick as that of an episode, but ends ear-
lier than the episode ends, it will withdraw all versions wholly or
partially included within its timespan. If one version is partially
within the timespan, the temporal delete will replace the part
of that withdrawn version not within its timespan. In either case,
the result
is a {shorten backwards}
transformation on that
episode.
[Starts
-1
]. If a temporal delete transaction's effective time
begins on the same clock tick as that of an episode, but ends
after the episode ends, the transaction will {erase} the episode;
and, in addition, it will withdraw all other versions, for the same
object, that are wholly or partially included within its timespan.
Those other versions will exist within one or more later
episodes. On any of those episodes wholly included within the
transaction's timespan, there will be an {erase} transformation
on them, as well. The last episode within the transaction's
timespan may be wholly or partially included within that
timespan. If it is wholly contained, there will be an {erase} trans-
formation on it. Otherwise, there will be a {shorten forwards}
transformation. If the end of the transaction's timespan does
not fall on a version effective time boundary, then the temporal
delete will replace the part of that withdrawn version that is
not within its timespan.
