Network and Hierarchical Data
Models
References:
Elmasri (2nd ed.): Chap 10
Date, 5th ed.: Appendix C.
Revised: 25-8-2004
Objective: Basic understanding of
Network and hierarchical datamodels
that were used before the advent of
relational databases.
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The Network Data Model (NDM)
Networks are natural ways of
representing relationships among
objects in maths, chemistry, sociology
etc.
Networks can be represented by a
mathematical structure called a directed
graph (digraph)
A digraph consists of points or nodes
connected by arrows or edges.
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Conference on Data Systems
Languages (CODASYL), an
organisation of representatives from
major hardware and software vendors
and users, developed and standardised
COBOL language and in late 1960s set
up the Database Task Group (DBTG) to
develop standards for DBMS.
DBTG was heavily influenced by
architecture of Integrated Data Store
(IDS).
Fundamental definition of NDM referred
to as CODASYL DBTG model.
NDM nodes are data record types and
edges represent one to one or one to
many relationships (links)
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Concepts and definitions
NDM conforms to ANSI/SPARC 3-level
architecture.
Conceptual level (logical view of data
and relationships) is called the schema.
External level (users’ views) is called
the subschema.
Internal level (physical storage details)
is implicit in the implementation.
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NDM has two basic data structures:
Record Types - collections of logically
related data items.
Sets (cosets or links) represent one to
one and one to many relationships
(N.B.: NOT the same as a mathematical
set).
Each set has one record type called the
owner and the other record type called
the member.
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A set has a name and is constructed
from owner and member record types
where arrow points from owner to
member. Whole data structure is
constructed from these sets.
Instances or occurrences are actual
record values in a data structure.
NDM allows only simple networks in
which all relationships are one-to-one or
one-to-many but many-to-many
relationships can be transformed into
one- to-many.
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Query Language
The NDM DML statements must be
embedded in a host language.
Operators process one record at a time.
Each user or application program has a
User Working Area (UWA).
Records in the subschema for that user
or application are stored in the UWA
together with:
Record templates - formats used for
records that are read into the UWA.
Status flags - variables used to denote
the success or failure of last operation
currency indicators - pointers to current
record(s)
Current (record) of (the) Run Unit contains the address of the most
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recently accessed record occurrence no
matter what type or what set it
participates in (Run Unit refers to the
users program).
Current of Record Type, R - contains
the address of the most recently
accessed record occurrence of record
type R
Current of Set Type, S - contains the
address of the most recently accessed
record participating in set type S (owner
or member)
Retrieval and Update
Navigation: FIND (a record)
Retrieval: GET
Update: ERASE, STORE, MODIFY
Set Update: CONNECT,
DISCONNECT, RECONNECT
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Hier and Network
Loads the record into a record template in UWA.
MOVE 105 TO CUST-ID IN CUSTOMER
FIND FIRST CUSTOMER USING CUST-ID
GET CUSTOMER
Get info of cutomer 105.
Queries in COBOL
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Hier and Network
Note: On EOF, DB-STATUS != 0
MOVE 0 TO ACCOUNT-BALANCE IN CUSTOMER
FIND FOR UPDATE FIRST CUSTOMER USING
ACCOUNT-BALANCE
WHILE DB-STATUS = 0
ERASE CUSTOMER
FIND FOR UPDATE NEXT CUSTOMER USING
ACCOUNT-BALANCE
END WHILE
Delete customer accounts with zero balance.
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Hier and Network
MOVE 231 TO INVNO IN INVOICE
MOVE "7/7/94" TO DATE IN INVOICE
MOVE 100.00 TO INVOICE-AMOUNT IN INVOICE
STORE INVOICE
MOVE 431 TO CUST-ID IN CUSTOMER
FIND FIRST CUSTOMER USING CUST-ID
CONNECT INVOICE TO CUSTOMER
Create a new invoice for $100.00 for customer 431.
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Hier and Network
This connects customer 431 with invoice 510 and disconnects
customer 425 from invoice 510.
MOVE 510 TO INVNO IN INVOICE
FIND FIRST INVOICE USING INVNO
MOVE 431 TO CUST-ID IN CUSTOMER
FIND FIRST CUSTOMER USING CUST-ID
RECONNECT INVOICE IN CUST-INV
Change invoice 510 from customer 425 to customer 431.
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Hierarchical Data Model (HDM)
This grew out of practice from IBM’s
Information Management System (IMS)
with out any standard or mathematical
basis.
IMS is a widely used DBMS developed
to support the Apollo moon project.
In some sence, HDM is a special case
of NDM, and is organised using tree
data structures in which a child can
have only one parent.
A database is represented as an
ordered set of trees.
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Basic Ideas
There are two basic data types.
Segment Types correspond to record
type of NDM.
PCR Types (Parent-Child Relationship
Types) represent a relationship between
a parent and child.
There is a single segment called the
root segment which does not participate
as a child in any PCR type.
Apart from the root segment, every
segment participates as a child in
exactly one PCR type.
A segment can participate as a parent
segment in more than one PCR type.
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For any segment type A, there is a
single path in the tree from the root to A.
The records along this path are called
ancestors of A.
A is a dependent segment of all
segments on that path, including the
root segment.
Database records (occurrences)
contain actual values.
Each occurrence is a distinct tree (in an
ordered set of trees) containing one root
segment.
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IMS DML (DL/1)
The data manipulation language of IMS
is called Data Language 1 (DL/1).
Database description (DBD) defines
format, length and location of each data
item and position in tree structure.
System maintains a program work area
for each application which contains,
Segment templates - segment format.
Currency pointers - for each database
tree; contains the address of segment in
tree most recently accessed.
Status flags - indicate outcome of last
db operation, e.g. successful
status
= 0.
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Common Commands
The general syntax is,
Command <seg name> <qual>
GET UNIQUE (GU) - retrieve first
segment that satisfies condition.
GET NEXT (GN) - get next segment.
GET NEXT WITHIN PARENT (GNP) retrieve next segment only within
current parent segment.
INSERT (ISRT) - add a segment.
REPLACE (REPL) - modify a value in a
segment field.
DELETE (DLET) - delete a segment.
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GU DEPARTMENT
EMPLOYEE (EMPNAME = ’Steve Smith’)
Get employee record for Steve Smith; don’t know where he is
assigned (requires sequential search of DEPARTMENT
segments).
GU DEPARTMENT (DEPTNAME = ’Marketing’)
EMPLOYEE (EMPNAME = ’Steve Smith’)
Get employee record for Steve Smith who works in Marketing.
(Retrieves a dependent segment by specifying path.)
Examples
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Deletes linked segments as well.
GHU DEPARTMENT (DEPTNAME = ’Marketing’)
EMPLOYEE (EMPNAME = ’Irving Schatz’)
DLET
Delete Irving Schatz’s record
GHU DEPARTMENT (DEPTNAME = ’Marketing’)
EMPLOYEE (EMPNAME = ’Irving Schatz’)
MOVE 25000 TO SALARY
REPL
Change Irving Schatz’s salary, in Marketing, to $25,000
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