Database System is a computerized record keeping system.

Transcription

1 Karlstad University Department of Information Systems Adapted for a textbook by Date C. J. An Introduction to Database Systems Pearson Addison Wesley, Database Management Remigijus GUSTAS Phone: Remigijus.Gustas@kau.se Database System is a computerized record keeping system. What is a Database? Repository for a collection of computerized data files Users can request the system to perform operations involving data files: ADD new or REMOVE existing file, INSERT or DELETE data into/from file, UPDATE data, RETRIEVE data. 1-2 Example of Table CELLAR primary key column Attributes BIN# PRODUCER BOTTLES YEAR row 2 Buena Vista (tuple) 21 Geyser Peak Jekel 5 Fumé Blanc

2 Retrieval SELECT, BOTTLES, YEAR FROM CELLAR WHERE YEAR = ; Bottles Year 1-4 DELETE FROM CELLAR WHERE BIN# = 2 ; BIN # PRODUCER BOTTLES YEAR 2 Buena Vista Geyser Peak Jekel 5 Fumé Blanc INSERT INTO CELLAR ( BIN#,, PRODUCER, YEAR, BOTTLES ) VALUES ( 11, Rioja, Aster,, 1 ) ; BIN # PRODUCER BOTTLES YEAR 11 Rioja Aster 1 21 Geyser Peak Jekel 5 Fumé Blanc

3 UPDATE CELLAR SET BOTTLES = 4 WHERE BIN# = 11 ; BIN # PRODUCER BOTTLES YEAR 11 Rioja Aster 1 21 Geyser Peak Jekel 5 Fumé Blanc Database System Purpose: Store data, Allow users to retrieve and update information. Data is stored in the database. Information refers to the meaning of data (as understood by some user). Database system components: Data, hardware, software, users 1-8 Database system component - Data May support single or many users Multi-User system many users can access the database at the same time Many users in organizations Data is integrated (not redundant) Data is shared among different users Different users will require different views 1-9

4 Database system component - Hardware Data is stored on magnetic Disks Direct access to portions of data Rapid I/O devices Data operated (by hardware processor) in the main memory 1-10 Database system component - Software Synonyms: Database manager Database server Database management system (DBMS) All user requests are handled by the DBMS DBMS are provided by specific vendor: Oracle 9i, Ingres, Informix, Sybase, DB Database system component - Users Application programmers End users Access database interactively Query language processor is used to query database (SQL, for instance SELECT operator) Database Administrators (technical people) Responsible for implementing the data administrator s decisions, Data administrator decides what data should be stored, establish policies for maintaining and dealing with data 1-4

5 1-1 What is a Database? Collection of persistent data Not transient Can be removed from a database only by some explicit request to DBMS Collection of true propositions Made up of entities, relationships, properties Implements a data model Implementation is a physical realisation on a real machine Introduction to Entity-Relationship (E-R) Modeling Entity-Relationship (E-R) Diagram A detailed, logical representation of the entities, associations and data elements for an organization or business Notation uses three main constructs Data entities Relationships Attributes 1-14 Association between the instances of one or more entity types Person, place, object, event or concept about which data is to be maintained Entity type: collection of entities with common characteristics Entity instance: single entity named property or characteristic of an entity

6 Entity and Attribute Example Simple attributes Identifier attribute each employee has a unique ID. Multivalued attribute an employee may have more than one skill Degree of Relationship Degree: number of entity types that participate in a relationship Three cases Unary: between two instances of one entity type Binary: between the instances of two entity types Ternary: among the instances of three entity types 1-17 Relationships of Different Degrees

7 Binary Relationship Examples 1-19 Cardinality The number of instances of entity B that can or must be associated with each instance of entity A Minimum Cardinality The minimum number of instances of entity B that may be associated with each instance of entity A Maximum Cardinality The maximum number of instances of entity B that may be associated with each instance of entity A Mandatory vs. Optional Cardinalities Specifies whether an instance must exist or can be absent in the relationship 1-20 A relationship that itself is related to other entities via another relationship must be represented as an associative entity

8 Benefits of Databases Shared data Reduced redundancy Reduced data inconsistency (corollary) Transaction support (all operations or none) Transaction is a logical unit of work involving several database update operations Support for data integrity (integrity constraints) Security enforcement Conflicting requirements can balanced Standards can be enforced 1-22 Data Independence Data dependence means that data access techniques are built into the application code. It is impossible to change the physical representation without affecting the application. Database separates logical and physical representation of data Allows changes to application programs without changing the structure of the underlying data and vice versa Applications do not depend on physical representation Database should be able to grow without impairing existing applications 1-2 Fields, Records and Files Stored field is the smallest unit of stored data (occurrences or instances of field type) Stored record is a collection of related stored fields (occurrence consists of a group of related field occurrences) Stored file is the collection of all occurrences of stored records Old systems, e.g., COBOL applications, directly connect to data formats

9 Relational Systems Most important innovation in database history Principles were originally laid down by (researcher at IBM) E F Codd in Based on logic and mathematics Data is perceived as tables, only Operators derive new tables from existing A table is a relation, mathematically Not pointer based (that are visible to the user) 1-25 Not Relational Systems Hierarchical Network Object Object/Relational Multi-dimensional Deductive Semistructured