<s>
A	O
slowly	B-Application
changing	I-Application
dimension	I-Application
(	O
SCD	O
)	O
in	O
data	B-General_Concept
management	I-General_Concept
and	O
data	O
warehousing	O
is	O
a	O
dimension	O
which	O
contains	O
relatively	O
static	O
data	O
which	O
can	O
change	O
slowly	O
but	O
unpredictably	O
,	O
rather	O
than	O
according	O
to	O
a	O
regular	O
schedule	O
.	O
</s>
<s>
Some	O
examples	O
of	O
typical	O
slowly	B-Application
changing	I-Application
dimensions	I-Application
are	O
entities	O
such	O
as	O
names	O
of	O
geographical	O
locations	O
,	O
customers	O
,	O
or	O
products	O
.	O
</s>
<s>
Some	O
scenarios	O
can	O
cause	O
referential	B-General_Concept
integrity	I-General_Concept
problems	O
.	O
</s>
<s>
This	O
fact	O
table	O
would	O
be	O
linked	O
to	O
dimensions	O
by	O
means	O
of	O
foreign	B-Application
keys	I-Application
.	O
</s>
<s>
In	O
the	O
above	O
example	O
,	O
Supplier_Code	O
is	O
the	O
natural	B-Application
key	I-Application
and	O
Supplier_Key	O
is	O
a	O
surrogate	B-Application
key	I-Application
.	O
</s>
<s>
Technically	O
,	O
the	O
surrogate	B-Application
key	I-Application
is	O
not	O
necessary	O
,	O
since	O
the	O
row	O
will	O
be	O
unique	O
by	O
the	O
natural	B-Application
key	I-Application
(	O
Supplier_Code	O
)	O
.	O
</s>
<s>
This	O
method	O
tracks	O
historical	O
data	O
by	O
creating	O
multiple	O
records	O
for	O
a	O
given	O
natural	B-Application
key	I-Application
in	O
the	O
dimensional	O
tables	O
with	O
separate	O
surrogate	B-Application
keys	I-Application
and/or	O
different	O
version	O
numbers	O
.	O
</s>
<s>
Transactions	O
that	O
reference	O
a	O
particular	O
surrogate	B-Application
key	I-Application
(	O
Supplier_Key	O
)	O
are	O
then	O
permanently	O
bound	O
to	O
the	O
time	O
slices	O
defined	O
by	O
that	O
row	O
of	O
the	O
slowly	B-Application
changing	I-Application
dimension	I-Application
table	O
.	O
</s>
<s>
To	O
reference	O
the	O
entity	O
via	O
the	O
natural	B-Application
key	I-Application
,	O
it	O
is	O
necessary	O
to	O
remove	O
the	O
unique	O
constraint	O
making	O
referential	B-General_Concept
integrity	I-General_Concept
by	O
DBMS	O
impossible	O
.	O
</s>
<s>
Both	O
the	O
surrogate	B-Application
keys	I-Application
are	O
referenced	O
in	O
the	O
fact	O
table	O
to	O
enhance	O
query	O
performance	O
.	O
</s>
<s>
This	O
method	O
resembles	O
how	O
database	O
audit	O
tables	O
and	O
change	B-General_Concept
data	I-General_Concept
capture	I-General_Concept
techniques	O
function	O
.	O
</s>
<s>
The	O
type	O
5	O
slowly	B-Application
changing	I-Application
dimension	I-Application
allows	O
the	O
currently-assigned	O
mini-dimension	O
attribute	O
values	O
to	O
be	O
accessed	O
along	O
with	O
the	O
base	O
dimension	O
's	O
others	O
without	O
linking	O
through	O
a	O
fact	O
table	O
.	O
</s>
<s>
In	O
many	O
Type	O
2	O
and	O
Type	O
6	O
SCD	O
implementations	O
,	O
the	O
surrogate	B-Application
key	I-Application
from	O
the	O
dimension	O
is	O
put	O
into	O
the	O
fact	O
table	O
in	O
place	O
of	O
the	O
natural	B-Application
key	I-Application
when	O
the	O
fact	O
data	O
is	O
loaded	O
into	O
the	O
data	O
repository	O
.	O
</s>
<s>
The	O
surrogate	B-Application
key	I-Application
is	O
selected	O
for	O
a	O
given	O
fact	O
record	O
based	O
on	O
its	O
effective	O
date	O
and	O
the	O
Start_Date	O
and	O
End_Date	O
from	O
the	O
dimension	O
table	O
.	O
</s>
<s>
Having	O
a	O
Type	O
2	O
surrogate	B-Application
key	I-Application
for	O
each	O
time	O
slice	O
can	O
cause	O
problems	O
if	O
the	O
dimension	O
is	O
subject	O
to	O
change	O
.	O
</s>
<s>
A	O
pure	O
Type	O
6	O
implementation	O
does	O
not	O
use	O
this	O
,	O
but	O
uses	O
a	O
surrogate	B-Application
key	I-Application
for	O
each	O
master	B-General_Concept
data	I-General_Concept
item	O
(	O
e.g.	O
</s>
<s>
each	O
unique	O
supplier	O
has	O
a	O
single	O
surrogate	B-Application
key	I-Application
)	O
.	O
</s>
<s>
This	O
avoids	O
any	O
changes	O
in	O
the	O
master	B-General_Concept
data	I-General_Concept
having	O
an	O
impact	O
on	O
the	O
existing	O
transaction	O
data	O
.	O
</s>
<s>
Referential	B-General_Concept
integrity	I-General_Concept
by	O
DBMS	O
is	O
now	O
possible	O
,	O
but	O
one	O
cannot	O
use	O
Supplier_Code	O
as	O
foreign	B-Application
key	I-Application
on	O
Product	O
table	O
and	O
using	O
Supplier_Key	O
as	O
foreign	B-Application
key	I-Application
each	O
product	O
is	O
tied	O
on	O
specific	O
time	O
slice	O
.	O
</s>
<s>
An	O
alternative	O
implementation	O
is	O
to	O
place	O
both	O
the	O
surrogate	B-Application
key	I-Application
and	O
the	O
natural	B-Application
key	I-Application
into	O
the	O
fact	O
table	O
.	O
</s>
<s>
Referential	B-General_Concept
integrity	I-General_Concept
by	O
DBMS	O
is	O
not	O
possible	O
since	O
there	O
is	O
not	O
a	O
unique	O
key	O
to	O
create	O
the	O
relationship	O
.	O
</s>
<s>
Some	O
Business	B-General_Concept
Intelligence	I-General_Concept
tools	O
do	O
not	O
handle	O
generating	O
complex	O
joins	O
well	O
.	O
</s>
