Databases Reference
In-Depth Information
Time Period Relationships
Along a Common Timeline
Excludes
Meets
|-----|-----|
Figure 14.12 P
1
[meets] P
2
.
The predicate for this relationship, as it holds between two
time periods expressed as pairs of dates using the closed-open
convention, is:
(eff_end_dt
1
¼
eff_beg_dt
2
)
It says that after P
1
ends, P
2
begins on the very next clock tick.
There is no clock tick gap between them. Say that both dates are
5/13/2004. This means that the last clock tick in P
1
is 5/12/2004
and the first clock tick in P
2
is 5/13/2004, and so there are no
clock ticks between the two time periods.
The inverse of this relationship is: P
2
[meets
1
]P
1
. In the super-
scripted relationship, the first time period is the later one. The pred-
icate for this relationship, as it holds between two time periods
expressed as pairs of dates using the closed-open convention, is:
(eff_beg_dt
1
¼
eff_end_dt
2
)
It says that before P
1
begins, P
2
ends on the previous clock
tick. There is no clock tick gap between them.
This is a particularly useful relationship because it defines a
collection of versions of the same object that belong to the same
episode. Every adjacent pair of versions of the same object that
do not share any clock ticks, i.e. in which neither includes the
other, and which also do not have a single clock tick between
them, belong to the same episode. The earlier version of the pair
meets the later one; the later version is met by the earlier one.
Throughout this topic, if it isn't important which of two time
periods that meet come first, we will simply say that the two
time periods are contiguous.
Consider the following request for information: which policies
ended immediately before the Diabetes Management Wellness
Program for 2009 began?
