الوحدة الثانية من قواعد البيانات

SDL is typically realized via DBMS commands provided to the DBA and database designers

The actual data stored in a database at a particular moment in time.

remove a record or data item

Provide concepts that fall between the above two, used by many commercial DBMS implementations

changes very infrequently

Used to specify database retrievals and updates

Physical

Physical

describe the structure and constraints for the whole database for a community of users.

Relational DBMS servers are often called

Conceptual

changes every time the database is updated.

Used to specify the conceptual schema of a database.

Data extracted from the internal DBMS level is reformatted to match the user’s external view

Same type of databases connected

When a schema at a lower level is changed The higher-level schemas themselves are unchanged.

can be applied directly (called a query language)

multiple computers, multiple DBs

Usually uses the same data model as the conceptual schema.

May be used in a standalone way or may be embedded in a programming language

دا تعريف ايه؟

Changes to constraints

Combines everything into single system including DBMS software, hardware, application programs, andUI

High-Level or Non-procedural Languages

describe the various user views.

Common for Web applications

SQL

The capacity to change the internal schema without having to change the conceptual schema or external schema.

Provide concepts that describe details of how data is stored in the computer

intension

Used by the DBA

Programs refer to an internal schema, and are mapped by the DBMS to the internal schema for execution.

User can't connect through a remote terminal in a Centralized DBMS

In some DBMSs, separate (SDL) and (VDL) are used to define internal and external schemas.

Includes descriptions of the database structure, data types, and the constraints on the database

When a schema at a lower level is changed, the application programs need not be changed

Different type of databases connected

Typically uses a physical data model

must be embedded in a programming language

When a schema at a lower level is changed,nothing has to be changed in a DBMS that fully supports data independence.

Uses conseptual or implementation data model.

Refers to the content of a database at a moment in time.

A state that satisfies the structure and constraints of the database.

Expand the DB

UI and apps

Conceptual

In many DBMSs, the ____ is also used to define internal and external schemas (views).

s (e.g. relational data models used in many commercial systems).

Provide concepts that are close to the way many users perceive data

extension

Reorganization of physical files

add a record or data item

occurrence or snapshot

Refers to the database state when it is initially loaded into the system

relational language SQL

Reduce the DB

(e.g indexes).

The description of a database

ER

the internal schema may be changed when certain file structures are reorganized or new indexes are created

Uses implementation data model.

single computer with one database

Mappings among schema levels are needed to transform requests and data.

be embedded in a generalpurpose programming language (host language), such as C++, or Java.

SQL

Implementation

The capacity to change the conceptual schema without having to change the external schemas and blabla

An illustrative display of (most aspects of) a database schema.

describe physical storage structures and access paths

Used by database designers

Specialized Servers with Specialized functions

Query and transaction functionality

Two-tier Architecture Can Enhance Security:

Low Level or Procedural Languages: