The TSQL2 language, extends SQL for querying valid
time, transaction time, and bitemporal relational databases.
TEMPORAL DATABASES
In a temporal database, the conditions may involve
time in addition to attributes.
A pure time condition involves only time
PURE TIME CONDITIONS:
Suppose a
user wants to select all employee versions that were valid at any point during
1997.
SELECT NAME
FROM EMPLOYEE
WHERE [t.VST, t.VET] OVERLAPS [1997-01-01,
1997-12-31
ATTRIBUTE AND TIME CONDITION
Suppose we wish to retrieve all EMP_VT tuple
versions t for
employees who worked in department 5 at any time
during 1997.
SELECT
* FROM EMPLOYEE WHERE
([t.VST,
t.VET] OVERLAPS [1997-01-01, 1997-12-31]) AND (t.DNO = 5)
3.3 Common operations used in queries
[t.VST, t.VET] INCLUDES [t1, t2] Equivalent
to (t1>= t.VST AND t2<= 1 t.VET )
[t.VST, t.VET] INCLUDED_IN [t1, t2] Equivalent to (t1 <=1
t.VST AND t2>= t.VET)
[t.VST, t.VET] OVERLAPS [t1, t2] Equivalent to (t1 1<= t.VET AND t2>= t.VST)
[t.VST, t.VET] BEFORE [t1, t2] Equivalent to
(t1 >=t.VET)
[t.VST, t.VET] AFTER [t1, t2] Equivalent
to (t2<= t . VST)
[t.VST, t.VET] MEETS_BEFORE [t1, t2] Equivalent to (t1
= t.VET + 1 )
[t.VST, t.VET] MEETS_AFTER [t1, t2] Equivalent to (t2
+ 1 = t.VST )
For any two time periods [t1, t2] and [t3, t4] in a
temporal element, the
following three conditions must hold:
• [t1, t2] intersection [t3, t4] is empty.
•
t3 is not the time point following t2 in the given granularity.
•
t1 is not the time point following t4 in the given granularity.
