THE NETWORK DATA MODEL

In the network data model, the database consists of a collection of set-type occurrences.

Each set-type occurrence has one occurrence of OWNER RECORD, with zero or more occurrences of MEMBER RECORDS.

The member sets belonging to different owners are disjoint.

To define a network database one needs to define:
(a) the database record types which consist of data items, and
(b) the set-types.

Network Set-Type Occurence

CONCEPT OF CURRENCY

For the network approach, since the application programmer must navigate through the database, the concept of CURRENCY is used to determine the current run-unit record occurrences, and set-type occurrences.

suppose the commands are:

MOVE 'S4' to SNO IN S
FIND S RECORD
FIND FIRST SP RECORD OF S-SP SET
FIND OWNER IN P-SP OF CURRENT OF S-SP SET

At the end of execution of the above commands, we have:

Current of run-unit: P 'P2'
Current S occurrence: S 'S4'
Current P occurrence: P 'P2'
Current SP occurrence: SP 'S4, P2, 2'
Current of set S-SP: SP 'S4, P2, 2'
Current of set P-SP: P 'P2'
Current S-SP occurrence: owned by S 'S4'
Current P-SP occurrence: owned by P 'P2'
Current of Basic-Data-Area: P 'P2'
Current of Link-Data-Area: SP 'S4, P2, 2'

Example Supplier-Parts Database

USER WORK AREA (UWA)

A set of program variables, declared in the host program, to communicate the contents of individual records between the DBMX and the host program.

For each record type in the database schema, a corresponding program variable with the same format should be declared in the host program.

NETWORK DATABASE MANIPULATION LANGUAGE

The major commands are: FIND, GET, MODIFY, INSERT, REMOVE, DELETE, and STORE.

(1) FIND command is used to locate an existing record occurrence, and establish it as the current of the run-unit, and also the current of the appropriate area, appropriate record type, and all sets in which it participates. The simplest form of the FIND command is as follows.

MOVE 'S4' TO SNO IN S.
FIND S RECORD.

The first statement constructs a prototype S record in the user's work area, which is then used to find the S record in the database.

DML Commands

(2) GET command is used to retrieve the current of run-unit. For example, the commands

MOVE 'S4' TO SNO IN S.
FIND S RECORD.
GET S; SNAME, CITY.

retrieve details of supplier 'S4'. The first two commands are the same as in previous example. The last command retrieves the current of the run-unit (the S record for 'S4') into the UWA.

DML Commands

(3) MODIFY command is used to update the current of run-unit, as illustrated in the follow example.

MOVE 'S4' TO SNO IN S.
FIND S RECORD.
GET S; STATUS.
ADD 10 TO STATUS IN S.
MODIFY S; STATUS.

DML Commands

(4) INSERT command inserts current of run-unit into one or more set occurrences. For example, the following commands insert the EMP record occurrence for employee 'E6' as a member into the DEPT-EMP occurrence owned by DEPT occurrence 'D1'.

MOVE 'D1' TO DNO IN DEPT.
FIND DEPT RECORD.
MOVE 'E6' TO ENO IN EMP.
FIND EMP RECORD.
INSERT EMP INTO DEPT-EMP.

DML Commands

(5) REMOVE command removes current of run-unit from one or more set occurrences. An example follows.

MOVE 'D1' TO DNO IN DEPT.
FIND DEPT RECORD.
MOVE 'E6' TO ENO IN EMP.
FIND EMP RECORD.
REMOVE EMP FROM DEPT-EMP.

DML Commands

(6) DELETE command deletes the current of run-unit, which may be owner records.

MOVE 'S4' TO SNO IN S.
FIND S RECORD.
DELETE S [ONLY, SELECTIVE, ALL].

The keyword after the DELETE S clause specifies the side effects of the deletion command. If no keyword is specified, then S is deleted only if S is not owner of any nonempty set occurrences. If "ONLY" is specified, then S is deleted together with all MANDATORY member occurrences. If "SELECTIVE" is specified, S is deleted together with all MANDATORY member occurrences and other member occurrences that are not in any other set occurrences. IF "ALL" is specified, then S is deleted together with all of its members.

DML Commands

(7) STORE command creates a new record occurrence which could be a link record, and establishes it as the current of the run-unit. For example, to crate a new SP record 'S5, P6, 7', the commands are:

MOVE 'S5' TO SNO IN SP.
MOVE 'P6' TO PNO IN SP.
MOVE '7' TO QTY IN SP.
MOVE 'S5' TO SNO IN S.
MOVE 'P6' TO PNO IN P.
STORE SP.

MEMBERSHIP CLASS

It specifies the semantics of database operations.

MANDATORY: Record can only be removed when its owner is removed (i.e., the REMOVE command may not be used the DELETE command must be used).

OPTIONAL: The record can be removed or deleted.

AUTOMATIC: It can be added to a set with the correct owner using the STORE command.

MANUAL: It will not be automatically added to the set with the correct owner, and the INSERT command is necessary.

LOCATION MODE

In defining a network database, one has to define both the record types and the set types. When the record types are defined, one can also define the location mode, which specifies the access strategy for a record (an owner record or a member record). For example, in define the S record, the following clause may be added:

LOCATION MODE IS DIRECT S-KEY

and S-KEY is used as database key for addressing S records. The following clause,

LOCATION MODE IS CALC HASH-SNO USING SNO IN S

will establish SNO as the hash key for the hashing procedure HASH-SNO supplied by the application programmer.

INDIRECT ACCESS

DEPT record is accessed via DIV record and DIV-DEPT set-type.

The hierarchical relations among DIV, DEPT and EMP are as follows:

DIV ------------------>DEPT------------------->EMP
    DIV-DEPT set-type       DEPT-EMP set-type

The location mode for DEPT record can be specified as

LOCATION MODE IS VIA DIV-DEPT SET.

LOCATION MODE SPECIFIES ACCESS STRATGEY:
(a) direct
(b) calculated by hashing or indexing
(c) indirect via another set

RETRIEVAL EXAMPLES

(a) Find a record using a key. Suppose for record EMP, in its definition, ENO is defined as a key,

USAGE IS DATABASE-KEY.

Then to find EMP record, we can say

FIND EMP USING ENO.

RETRIEVAL EXAMPLES

(b) Find a record by a translation algorithm. Suppose EMP record has a LOCATION MODE in its definition,

LOCATION MODE IS CALC HASH-ENO
USING ENO IN EMP
DUPLICATES ARE NOT ALLOWED.

Then one can access an EMP record by setting up ENO, and issuing a FIND command.

MOVE 'E2' TO ENO IN EMP.
FIND EMP RECORD.

RETRIEVAL EXAMPLES

(c) Find next (first, last, prior) record for sequential processing. Suppose the desired DEPT record is found. To process EMP records, one can say,

FIND FIRST EMP RECORD OF DEPT-EMP SET.
FIND NEXT EMP RECORD OF DEPT-EMP SET.
FIND PRIOR EMP RECORD OF DEPT-EMP SET.

(d) Find owner of a record. For particular EMP record, one can find its owner record.

FIND OWNER IN DEPT-EMP OF CURRENT
OF DEPT-EMP SET.