Lecture_5

Databases. Main definitions.
This are resources to which the users having the corresponding access rights have to have access.
The database is the coordinated (consistent) data set which is falling into to a specific objective (tasks) together with logical communications between data.
Logical communications (schemes) define how one data correspond to others according to the DB logical model. In it the main difference of a DB from the file of data in which data are organized on physical sign in the form of a serial set of records consists.
The database management system represents a set of tools, and is more often - the realized programs intended for storage of the database, change of its contents, ensuring its safety and interaction with the user.

Databases. Trial functions.
Providing quick access to a DB.
Protection of a wholeness of a DB at hardware failures and program mistakes.
Differentiation of access rights and protection against unauthorized access to a DB.
Support of collaboration of several users from the common DB.
Data management in external memory;
Control of buffers of random access memory;
Management of transactions (operating sequences over a DB);
Journalizing and restoration of a DB after failures;
Maintaining of the DB languages.

Logical organization of databases.
The logical organization of databases is the logical model of subject domain reflecting three types of information about objects of subject domain:
data on objects of subject domain;
their properties;
their relations.
Objects on the scheme are defined by types of records, properties of objects - fields of records, the relations - define communications between types of records and fields.
Types of the DB logical models:
hierarchical;
network;
relational.

Data model definition
Data structure
Operations
Integrity constraints
schema – data description
instance – data value
data structure = schema + instance
data algebra = data structure + operations
data model = data algebra + integrity constraints

Field and Segment
Graphic notation:
TEACHER (Name, Post, Address)
Data field – elementary named data. Its instance is a data value.
Segment – is a named set of field names:
TEACHER
Name
Post
Address
Segment instance – ordered set of fields instances:
TEACHER
Johnson
professor
Kiev

Hierarchy Data Schema
TEACHER
Name
Post
Address
DEPARTMENT
Name
Head
Building
FACULTY
Name
Dean
Fund
FACULTY
DEPARTMENT
TEACHER

Hierarchy Data Schema

Hierarchy Schema Instance
FACULTY
DEPARTMENT
STUDENT
Informatics
CAD
Alex
Operating systems
Bobby
Diana Helen
Mary
John
Schema Schema instance

Fields and record types
Graphic notation:
TEACHER (Name, Post, Address)
Data field (item) – elementary named data. Its instance (occurrence) is a data value.
Record type – is a uniquely named collection of field names:
TEACHER
Name
Post
Address
Record type instance – ordered collection of fields instances:
TEACHER
Johnson
professor
Kiev

Keys
Two or more different records within a network data base may have duplicate values of all data items.
The Data Base Key (DBK) is a unique internal identifier of any record. DBK is automatically assigned to any record when it is stored in DB and do not changed when a record is updated.
CALC key is an internal identifier of any record, that assigned to a record on the base of values of a specified record fields.

Example of the set type and instance
DEPARTMENT
TEACHER
SUBJECT
logics
DBMS
С language
Peter
John
Ann
SE
Department Structure
Set owner
Set members
Set type Set instance
Set name

Multilevel Hierarchy
DEPARTMENT
TEACHER
SUBJECT
Department Structure
BOOK
ARTICLE
Scientific Works
Owner of the set «Department Structure»
Members of the set «Department Structure»
Owner of the set «Scientific Works»
Members of the set «Scientific Works»

Possible categories
of network schemas

Nonformal introduction
to relations
Form of representation:
As a table
By using a condition
Relation is an association between any number of entities.
Like subject Is more Supply
Who What First Second Who Whom What Q-ty
John DBMS 5 3 П1 К7 table 200
Peter С 17 5 П3 К14 door 150
Ann XML 2 1 П18 К9 window 1000