Lecture 4. Network Data Model

Cycles and loops

Lecture 4. Network Data Model

Possible categories of network schemas

Hierarchical structures

Network structures

Loops

(one level cycles)

Lecture 4. Network Data Model

Application and DBMS

User Work Area (UWA) contains:

current state indicators,

records,

error status.

Lecture 4. Network Data Model

Network data model operations

Data selection (Query Language - QL):

FIND - locates a described record occurrence

GET - reads a record occurrence from the database

Data manipulation (Data Manipulation Language-DML):

STORE - puts a record occurrence into the database

INSERT - inserts member record into a set occurrence

REMOVE - removes member record from a set occurrence

DELETE - deletes current record from DB

MODIFY - updates current record

Lecture 4. Network Data Model

Data searching and selection

FIND Record location. A desired record occurrence is located. Located record occurrence becomes current. Current status indicator in UWA points to a located record occurrence

There is really set of FIND command that allow :

find the record by a key,

find any member record in a set occurrence,

scan a set occurrence for those member records having specified values in certain fields,

find an owner record of a given set occurrence.

GET Puts selected (current) record into UWA.

Lecture 4. Network Data Model

Fine the record by DB-key

Syntax:

FIND [record­name] RECORD BY DATABASE KEY [variable]

Example:

X=10;

FIND Faculty RECORD BY DATABASE KEY X;

If Error_Status=0 then GET Faculty;

Else Print "Not Found !";

Lecture 4. Network Data Model

Scanning a Set Occurrence

The set occurrence can be viewed as a “ring” consisting of the owner record and all of the member records. Member records are ordered within there owner record.

Lecture 4. Network Data Model

The next member record in the set occurrence

Syntax:

FIND NEXT [record­name] RECORD IN [set­name] SET

Example:

FIND NEXT Lecture RECORD IN Has SET;

Lecture 4. Network Data Model

The prior member record in the set occurrence

Syntax:

FIND PRIOR [record­name] RECORD IN [set­name] SET

Пример:

FIND PRIOR Lecture RECORD IN Has SET;