Databases Reference
In-Depth Information
The SQL written to fulfill this request is:
SELECT * FROM V_Allen_Example
WHERE pol_epis_end_dt
¼
wp_eff_beg_dt
P
1
[
excludes]
P
2
This not an Allen relationship. It is the node in our taxonomy
of Allen relationships which includes the [before], [before
1
],
[meets] and [meets
1
] relationships.
In the non-superscripted relationship, the first time period is
the earlier one. 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 P
2
starts either immediately after the end of P
1
,or
later than that. The idea behind it is that it includes every rela-
tionship in which P
1
and P
2
have no clock ticks in common
and in which P
1
is the earlier time period.
The inverse of this relationship is: P
1
[
excludes
1
]P
2
. The first
time period in this non-superscripted relationship is the later
one. 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
2
)
It says that P
2
ends either immediately before the start of P
1
,
or earlier than that. The idea behind it is that it includes every
relationship in which P
1
and P
2
have no clock ticks in common
and in which P
1
is the later time period.
All pairs of time periods that share no clock ticks satisfy one
or the other of these two predicates. So the predicate that
(eff_beg_dt
1
>¼
Time Period Relationships
Along a Common Timeline
Excludes
Before
Meets
|-----| |-----|
|-----|-----|
Figure 14.13 P
1
[excludes] P
2
.
