Information storage: Introduction of database 10/7/2004 Xiangming Mu

What is database system Database a collection of related data represents some aspect of the real world organized data for retrieval, maintenance has intended group of users Database Management System (DBMS) a collection of programs for creating and using a database (a software package) defines a DB: data type, structure, constraints constructs a DB: storing the data manipulates a DB: query, update, report Database System A Database + a DBMS

Data Models Conceptual (high-level) Data Model concepts: entity, attribute, relationship Entity-Relationship model (DBMS-independent) Representational (implementation-level) data represented by record structure i.e.) relational, network, hierarchical Physical (low-level) describes how data is stored in the disk

DB Schema vs. DB State Database Schema (intension) description of the database is specified during database design Database State (extension of the schema) current state of the database: a snapshot; it is an actual data instances (occurrences) in a DB changes over time by update initially, a database is empty state with no data then, populate (load) the database with data DBMS checks every state of the database does it satisfy the structure and constraints specified in the schema?

Three-Schema Architecture 1. An Internal Schema (internal level) physical storage structure, access paths uses a physical data model 2. A Conceptual Schema (conceptual level) describes whole database structure data types, constraints, user operations hides the details of storage structure uses conceptual/implementation data models 3. Multiple External Schemas (external level) each describes a part of the database for a particular user group and hides the rest support multiple views of a database same data model as the conceptual schema

Three-Schema Architecture (cont’) Mappings for multi-level DBMS –to transform a request specified at one level into the request at another level –access: external  conceptual  internal  DB –retrieve: DB  internal  conceptual  external Three-Schema Architecture advantage: true data independence disadvantage: overhead cost of mappings

DBMS Languages Data Definition Language (DDL) to define DB schemas VDL (View Definition Lang.): external schema SDL (Storage Definition Lang.): internal schema Data Manipulation Language (DML) to specify database requests: update, retrieval high-level DML: which data to retrieve low-level DML: how to retrieve it

Data Models Overview Relational Data Model database represented as a collection of tables each table stored as a separate file uses higher-level query language (SQL) Object Data Model database defined in terms of objects, their properties, and their operations classes: objects with same structure & behavior methods specify operations of each class Others Object-relational, hierarchical tree, XML, etc…

Conceptual Data Models Conceptual design of database applications, and DBMS-independent Including database structure and constraints --- Entity-Relationship (ER) Independent from its implementations (on representation level and physical level)

E-R Model Representation data as entities, attributes, and relationships E-R schema diagram (example)example Entity a "thing" (object, event) in the real world physical vs. conceptual existence: car vs. job has a set of properties to completely describe it Need a primary “key” attribute to identify a unique record of the entity Attributes –an entity has a value for each of its attributes

Relationship Relates two or more distinct entities with a specific meaning (example)example Relationship Instance an association among at least two entities belonging to one or more entity types ‘John works in Accounting Department’ Cardinality of a Relationship 1:1 (each entity can only participate once) 1:N (one entity can participate once, the other can participate many times) M:N (each entity can participate many times)

What is SQL SQL Structured Query Language, for commercial relational DBMSs a DBMS may have its own extensions SQL can create a relational DB schema create/modify/delete tables extract information from the DB (query) define views on the database specify constraints, security, authorization be embedded in a general-purpose programming language (C, PASCAL, Java)

SQL Features Data Definition Language (DDL) –allows database tables to be created or deleted. CREATE TABLE - creates a new database table ALTER TABLE - alters (changes) a database table DROP TABLE - deletes a database table Data Manipulation Language (DML) –modify and query database states –describes which data to retrieve, not how to retrieve it SELECT - extracts data from a database table UPDATE - updates data in a database table DELETE - deletes data from a database table INSERT INTO - inserts new data into a database table

An SQL example  SQL Query Example retrieve the names of all employees who work for the department number 4 SELECT FNAME, LNAME FROM EMPLOYEE WHERE DNO = 4;