Database Reference
In-Depth Information
According to (Morzy, 2004), the user has the
possibility to retrieve data from the following:
is the total number of versions that are valid be-
tween 'begin date' and 'end date', then B number
of partial results will be produced.
1)
Querying current DW version: this can be
done by using conventional SQL i.e. without
extension from query language. By default
each query is executed on the current version
of DW;
3)
Querying a single version: data will be re-
trieved from the specific version, mentioned
in the query.
2)
For querying multiple versions, the user has
to specify the versions involved for retrieval.
This demands extensions to clauses of the
query, from date 'begin date' to 'end date'.
This will specify valid time of the involved
versions. This way, data from all the versions
which are valid in the time period will be
retrieved. Generally, the query can be written
as,
Select
A
1
, A
2
… A
n
From
T
1
, T
2
… T
m
Where
Join
1
and Join
2
… Join
x
Group by
A
t
Version
= 'Vy.z'
Where Vy.z is the identifier of the version.
4)
merging results of partial queries: in this case
the user can access data from more than one
version and the result will not appear in the
form of partial results as stated in case 2.
Instead as a result of execution of the query,
a single output will be produced.
Select
A
1
, A
2
… A
n
From
T
1
, T
2
… T
m
Where
Join
1
and Join
2
… Join
x
Group by
A
t
Version begin date
'YYYY-MM-DD'
End date
'YYYY-MM-DD';
As an output to this query, a number of partial
results are produced. Each result represents output
of each version of the data warehouse. So, if 'B'
Select
A
1
, A
2
… A
n
From
T
1
, T
2
… T
m
Where
Join
1
and Join
2
… Join
x
Group by
A
t
Version begin date
'YYYY-MM-DD'
Figure 10. Multiversion data warehouse explorer (adopted from Morzy, 2004)
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