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Chapter 2 dbChapter 2 dbChapter 2 dbChapter 2 db.ppt
- 1. 1 Chapter 2 DATABASE SYSTEM CONCEPT AND ARCHITECTURE • DBMS Architecture – Tightly integrated systems architecture: the user interface + DBMS + data are located in one place. – Client-server system architecture: User Interface DBMS + Data Client Module Server Module Request Reply Ala Hamarsheh -- [email protected]
- 2. 2 • Data Model: a collection of concepts that used to describe the structure of database. Structure: data type, constraints, and relationships. – We use the data model to achieve some levels of data abstraction. – The data model should include a set of basic operations for retrievals and updates on the DB. • CATEGORIES OF DATA MODEL: – High level: provide concepts that are close to the way users perceive data (SQL). – Low Level: provide concepts that describe the details of how data is stored in the computer. – Representational (implementation) data model: provide concepts that may be understood by end users but that are not too far from the way data is organized within the computer (Entities) Ala Hamarsheh -- [email protected]
- 3. 3 • Schemas: – Any data model should distinguish between the description of the database and the database itself. The description of DB is called DB schema. – Schema diagram: displays the structure of each record type, BUT not the actual instances of records. StID SNAME CLASS MAJOR STUDENT Schema Construct This Schema will be changed Infrequently Ala Hamarsheh -- [email protected]
- 4. 4 – Database State: the current set of instances or occurrences. DB State will be changed frequently. Notes: When we define new database we specify DB schema. Every time an update operation is applied to DB, we get another DB state. DBMS responsible for ensuring that every state of the database is a valid state satisfies the structure and constrains specified in the database schema. DBMS stores the description of the schema constructs and constrains in the meta-data (catalog files). Schema is called intension whereas state is called extension. Ala Hamarsheh -- [email protected]
- 5. 5 THREE SCHEMA ARCHITECTURE: – Internal Level: to describe the data storage structure and access paths-typically uses physical data model. – Conceptual Schema: to describe the structure and constraints for the whole database.-uses implementational data model. – External Level: to describe the various user views. We need mapping among these levels. Ala Hamarsheh -- [email protected]
- 7. 7 DATA INDEPENDENCE: Changing the schema of one level of the DB system doesn't require to change the schema of the next higher levels. Two types of data independence: – Logical Data-Independence: the capacity to change the conceptual schema without having to change the external schema and their application programs. – Physical Data-Independence: the capacity to change the internal schema without having to change the conceptual schema. Ala Hamarsheh -- [email protected]
- 8. 8 DBMS LANGUAGES: – Data Definition language (DDL): used by DBA or DB designers to specify the conceptual schema of a DB. • In many DBMSs the DDL is also used to define internal and external schemas. • In some DBMSs separate storage definition language (SDL) and view definition language are used to define internal and external schema. – Data Manipulation Language (DML): used to specify database retrievals and updates. Ala Hamarsheh -- [email protected]
- 9. 9 DML Commands can be: 1. Embedded in a general purpose programming language (PL) like Java, Cobol, C++, .. Etc 2. Stand-alone, that applied directly (Query Language). TWO TYPES OF DML: – High-Level or Nonprocedural DML: DML statements must be identified within the program so that they can be extracted by the pre-compiler and accessed by DMBS. – Low-Level or Procedural DML: retrieves individual records from DB and process each one separately. Use Looping Ala Hamarsheh -- [email protected]