Databases Reference
In-Depth Information
Time Period Relationships
Along a Common Timeline
Intersects
Fills
Occupies
Aligns
Starts
|-----|
|------------|
Figure 14.2 P
1
[starts] P
2
.
two time periods expressed as pairs of dates using the closed-
open convention, is:
(eff_beg_dt
1
¼
eff_beg_dt
2
)
AND (eff_end_dt
1
>
eff_end_dt
2
)
It says that P
1
and P
2
begin at the same time, but that P
1
is the
last to end, and is therefore the longer of the two time periods.
Consider the following request for information: which policies
began when the Diabetes Management Wellness Program for
2009 began, but ended while that program was still going on?
The SQL written to fulfill this request is:
SELECT * FROM V_Allen_Example
WHERE pol_eff_beg_dt
¼
wp_eff_beg_dt
AND pol_epis_end_dt
<
wp_epis_end_dt
P
1
[finishes] P
2
This is a pair of relationships, one the inverse of the other.
In the non-superscripted relationship, the first time period is
the shorter one.
Figure 14.3
shows this relationship, and its place
