<s>
A	O
heterogeneous	B-General_Concept
database	I-General_Concept
system	I-General_Concept
is	O
an	O
automated	O
(	O
or	O
semi-automated	O
)	O
system	O
for	O
the	O
integration	B-General_Concept
of	O
heterogeneous	O
,	O
disparate	O
database	B-Application
management	I-Application
systems	I-Application
to	O
present	O
a	O
user	O
with	O
a	O
single	O
,	O
unified	O
query	O
interface	O
.	O
</s>
<s>
Heterogeneous	B-General_Concept
database	I-General_Concept
systems	I-General_Concept
(	O
HDBs	O
)	O
are	O
computational	O
models	O
and	O
software	O
implementations	O
that	O
provide	O
heterogeneous	B-General_Concept
database	I-General_Concept
integration	B-General_Concept
.	O
</s>
<s>
This	O
article	O
does	O
not	O
contain	O
details	O
of	O
distributed	B-Application
database	I-Application
management	I-Application
systems	I-Application
(	O
sometimes	O
known	O
as	O
federated	B-General_Concept
database	I-General_Concept
systems	I-General_Concept
)	O
.	O
</s>
<s>
Table	O
decompositions	O
may	O
vary	O
,	O
column	O
names	O
(	O
data	O
labels	O
)	O
may	O
be	O
different	O
(	O
but	O
have	O
the	O
same	O
semantics	O
)	O
,	O
data	B-Algorithm
encoding	I-Algorithm
schemes	O
may	O
vary	O
(	O
i.e.	O
,	O
should	O
a	O
measurement	O
scale	O
be	O
explicitly	O
included	O
in	O
a	O
field	O
or	O
should	O
it	O
be	O
implied	O
elsewhere	O
)	O
.	O
</s>
<s>
Data	O
across	O
constituent	O
databases	B-Application
may	O
be	O
related	O
but	O
different	O
.	O
</s>
<s>
Perhaps	O
a	O
database	B-Application
system	I-Application
must	O
be	O
able	O
to	O
integrate	O
genomic	O
and	O
proteomic	O
data	O
.	O
</s>
