Chapter 2 CIS548.101 Sungchul Hong Database Environment Chapter 2 CIS548.101 Sungchul Hong

The Three-Level ANSI-SPARC Architecture External Conceptual Internal <fig. 2.1>

Separation of Logical and Physical Presentation Each user should be able to access the same data, but have a different customized view of the data. Users should not have to deal directly with physical database storage details. DBA should be able to change the database storage structures without affecting the users’ views. The internal structure of the database should be unaffected by changes to the physical aspects of storage. The DBA should be able to change the conceptual structure of the database without affecting all users.

External Level Users’ view of the database. This level describes that part of the database that is relevant to each user. External views Different views may have different representations of the same data.

Conceptual Level The community view of the database. This level describes what data is stored in the database and the relationships among the data. Logical structure of entire database All entities, their attributes, and their relationships The constraints on the data Semantic information about the data Security and integrity information

Internal Level The physical representation of the database on the computer. This level describes how the data is stored in the database. Storage space allocation for data and indexes. Record descriptions for storage. Record placement Data compression and data encryption techniques.

Schemas Database schema External schemas (subschema) Conceptual schema Different views of the data Conceptual schema Describes all the entities, attributes,and relationships together with integrity constraints. Internal schema Complete description of the internal model, containing the definitions of stored records, the methods of representation, the data fields, and the indexes and hashing schemes used.

Mappings The DBMS is responsible for mapping between these three types of schema. Check consistency. Conceptual/internal mapping Find the actual record of a logical record External/conceptual mapping Map names in the user’s view on to the relevant part of the conceptual schema

Database Instance Database schema is not expected to change frequently. Database instance The data in the database at any particular point in time.

Data Independence Upper levels are unaffected by changes to lower level. Logical data independence Refers to the immunity of the external schemas to changes in the conceptual schema. Physical data independence Refers to the immunity of the conceptual schema to changes in the internal schema.

Data Definition Language A language that allows the DBA or user to describe and name the entities, attributes,and relationships required for the application, together with any associated integrity and security constraints. System catalog (meta data, data dictionary)

Data Definition Language (DDL) The DDL us used to define a schema or to modify an existing one. System catalog Data dictionary

The Data Manipulation Language (DML) A language that provides a set of operations to support the basic data manipulation operations on the data held in the database. Insertion, modification, retrieval, deletion of data Query language Procedural DMLs Non-procedural DML (SQL)

Fourth-Generation Languages (4GLs) What v.s. How Forms generator Report generators Graphics generators Application generators

Data Models and Conceptual Modeling An integrated collection of concepts for describing and manipulating data,relationships between data, and constraints on the data in a organization. Structural part Manipulation part Set of integrity rules Relational, network, hierarchical, Object-Oriented

Relational Data Model

Network Data Model

Hierarchical Data Model

Conceptual Modeling The process of constructing a model of the information use in an enterprise that is independent of implementations details such as the target DBMS, application programs, programming languages, or any other physical considerations.

Functions of a DBMS Data storage, retrieval, and update A user-accessible catalog (meta data) Transaction support (update salary) Concurrency control services Recovery service Authorization service Support for data communication

Functions of a DBMS (2) Integrity services Services to promote data independence Utility service Import, monitoring, statistical analysis, index reorganization, garbage collection.

Components of a DBMS Query processor Database manager File manager DML preprocessor DDL compiler Catalog manager Authorization control

Components of a DBMS (2) Command processor Integrity checker Query optimizer Transaction manager Scheduler Recovery manager Buffer manager

Multi-User DBMS Architectures Teleprocessing File-Server There is a large amount of network traffic A full copy of the DBMS is required on each workstation. Concurrency, recovery, and integrity control are more complex. Client-Server

Client-Server It enables wider access to existing databases. Increased performance. Different types of computers works in parallel. Hardware cost are reduced. Communication costs are reduced. Increased consistency – single server It maps on to open-systems architecture naturally.