<s>
A	O
hierarchical	B-General_Concept
database	I-General_Concept
model	I-General_Concept
is	O
a	O
data	B-Application
model	I-Application
in	O
which	O
the	O
data	O
are	O
organized	O
into	O
a	O
tree-like	O
structure	O
.	O
</s>
<s>
The	O
hierarchical	B-General_Concept
database	I-General_Concept
model	I-General_Concept
mandates	O
that	O
each	O
child	O
record	O
has	O
only	O
one	O
parent	O
,	O
whereas	O
each	O
parent	O
record	O
can	O
have	O
one	O
or	O
more	O
child	O
records	O
.	O
</s>
<s>
In	O
order	O
to	O
retrieve	O
data	O
from	O
a	O
hierarchical	B-General_Concept
database	I-General_Concept
,	O
the	O
whole	O
tree	B-Application
needs	O
to	O
be	O
traversed	O
starting	O
from	O
the	O
root	B-Application
node	I-Application
.	O
</s>
<s>
The	O
hierarchical	O
structure	O
was	O
developed	O
by	O
IBM	O
in	O
the	O
1960s	O
and	O
used	O
in	O
early	O
mainframe	O
DBMS	B-General_Concept
.	O
</s>
<s>
The	O
IBM	B-Application
Information	I-Application
Management	I-Application
System	I-Application
(	O
IMS	O
)	O
and	O
RDM	B-Application
Mobile	I-Application
are	O
examples	O
of	O
a	O
hierarchical	B-General_Concept
database	I-General_Concept
system	O
with	O
multiple	O
hierarchies	O
over	O
the	O
same	O
data	O
.	O
</s>
<s>
The	O
hierarchical	B-General_Concept
data	I-General_Concept
model	I-General_Concept
lost	O
traction	O
as	O
Codd	O
's	O
relational	B-Architecture
model	I-Architecture
became	O
the	O
de	O
facto	O
standard	O
used	O
by	O
virtually	O
all	O
mainstream	O
database	O
management	O
systems	O
.	O
</s>
<s>
A	O
relational-database	O
implementation	O
of	O
a	O
hierarchical	B-General_Concept
model	I-General_Concept
was	O
first	O
discussed	O
in	O
published	O
form	O
in	O
1992	O
(	O
see	O
also	O
nested	O
set	O
model	O
)	O
.	O
</s>
<s>
Hierarchical	B-General_Concept
data	I-General_Concept
organization	O
schemes	O
resurfaced	O
with	O
the	O
advent	O
of	O
XML	B-Protocol
in	O
the	O
late	O
1990s	O
(	O
see	O
also	O
XML	B-Application
database	I-Application
)	O
.	O
</s>
<s>
Currently	O
,	O
hierarchical	B-General_Concept
databases	I-General_Concept
are	O
still	O
widely	O
used	O
especially	O
in	O
applications	O
that	O
require	O
very	O
high	O
performance	O
and	O
availability	O
such	O
as	O
banking	O
,	O
health	O
care	O
,	O
and	O
telecommunications	O
.	O
</s>
<s>
One	O
of	O
the	O
most	O
widely	O
used	O
commercial	O
hierarchical	B-General_Concept
databases	I-General_Concept
is	O
IMS	O
.	O
</s>
<s>
Another	O
example	O
of	O
the	O
use	O
of	O
hierarchical	B-General_Concept
databases	I-General_Concept
is	O
Windows	B-Protocol
Registry	I-Protocol
in	O
the	O
Microsoft	B-Application
Windows	I-Application
operating	I-Application
systems	I-Application
.	O
</s>
<s>
An	O
organization	O
could	O
store	O
employee	O
information	O
in	O
a	O
table	B-Application
that	O
contains	O
attributes/columns	O
such	O
as	O
employee	O
number	O
,	O
first	O
name	O
,	O
last	O
name	O
,	O
and	O
department	O
number	O
.	O
</s>
<s>
The	O
organization	O
could	O
store	O
the	O
computer	O
hardware	O
information	O
in	O
a	O
separate	O
table	B-Application
that	O
includes	O
each	O
part	O
's	O
serial	O
number	O
,	O
type	O
,	O
and	O
the	O
employee	O
that	O
uses	O
it	O
.	O
</s>
<s>
+	O
employee	O
table	B-Application
EmpNo	O
First	O
Name	O
Last	O
Name	O
Dept	O
.	O
</s>
<s>
In	O
this	O
model	O
,	O
the	O
employee	O
data	O
table	B-Application
represents	O
the	O
"	O
parent	O
"	O
part	O
of	O
the	O
hierarchy	O
,	O
while	O
the	O
computer	O
table	B-Application
represents	O
the	O
"	O
child	O
"	O
part	O
of	O
the	O
hierarchy	O
.	O
</s>
<s>
In	O
contrast	O
to	O
tree	B-Data_Structure
structures	I-Data_Structure
usually	O
found	O
in	O
computer	O
software	O
algorithms	O
,	O
in	O
this	O
model	O
the	O
children	O
point	O
to	O
the	O
parents	O
.	O
</s>
<s>
In	O
Relational	O
database	O
terms	O
,	O
the	O
ReportsTo	O
column	O
is	O
a	O
foreign	B-Application
key	I-Application
referencing	O
the	O
EmpNo	O
column	O
.	O
</s>
<s>
If	O
the	O
"	O
child	O
"	O
data	O
type	O
were	O
different	O
,	O
it	O
would	O
be	O
in	O
a	O
different	O
table	B-Application
,	O
but	O
there	O
would	O
still	O
be	O
a	O
foreign	B-Application
key	I-Application
referencing	O
the	O
EmpNo	O
column	O
of	O
the	O
employees	O
table	B-Application
.	O
</s>
<s>
This	O
simple	O
model	O
is	O
commonly	O
known	O
as	O
the	O
adjacency	B-Data_Structure
list	I-Data_Structure
model	I-Data_Structure
and	O
was	O
introduced	O
by	O
Dr.	O
Edgar	O
F	O
.	O
Codd	O
after	O
initial	O
criticisms	O
surfaced	O
that	O
the	O
relational	B-Architecture
model	I-Architecture
could	O
not	O
model	O
hierarchical	B-General_Concept
data	I-General_Concept
.	O
</s>
<s>
However	O
,	O
the	O
model	O
is	O
only	O
a	O
special	O
case	O
of	O
a	O
general	O
adjacency	B-Data_Structure
list	I-Data_Structure
for	O
a	O
graph	O
.	O
</s>
