Chapter 1 Introduction n Purpose of Database Systems
Chapter 1: Introduction n Purpose of Database Systems n View of Data n Database Languages n Relational Databases n Database Design n Object-based and semistructured databases n Data Storage and Querying n Transaction Management n Database Architecture n Database Users and Administrators n Overall Structure n History of Database Systems Database System Concepts - 6 th Edition 1. 1
Database Management System (DBMS) n DBMS contains information about a particular enterprise Collection of interrelated data l Set of programs to access the data l An environment that is both convenient and efficient to use n Database Applications: l Banking: transactions l Airlines: reservations, schedules l Universities: registration, grades l 4 4 4 Add new students, instructors, and courses Register students for courses, and generate class rosters Assign grades to students, compute grade point averages (GPA) and generate transcripts Sales: customers, products, purchases l Online retailers: order tracking, customized recommendations l Manufacturing: production, inventory, orders, supply chain l Human resources: employee records, salaries, tax deductions n Databases touch all aspects of our lives l Database System Concepts - 6 th Edition 1. 2
Purpose of Database Systems n In the early days, database applications were built directly on top of file systems n Drawbacks of using file systems to store data: l Data redundancy and inconsistency 4 Multiple file formats, duplication of information in different files 4 Example: account (name, address, telephone, account-number, balance) loan (name, address, telephone, loan-number, amount) l Difficulty in accessing data 4 Need to write a new program to carry out each new task 4 Example: 輸出台北的顧客,再輸出台中的顧客 l Data isolation — multiple files and formats l Integrity problems 4 Integrity constraints (e. g. , account balance > 0) become “buried” in program code rather than being stated explicitly 4 Hard Database System Concepts - 6 th Edition to add new constraints or change existing ones 1. 3
Purpose of Database Systems (Cont. ) n Drawbacks of using file systems (cont. ) Atomicity of updates (see page 23) 4 Failures may leave database in an inconsistent state with partial updates carried out 4 Example: Transfer of funds (say $1000) from one account (say $5000) to another (say $2000) should either complete or not happen at all. l Concurrent access by multiple users 4 Concurrent access needed for performance 4 Uncontrolled concurrent accesses can lead to inconsistencies l – Example: Two people reading a balance (say $100) and updating it by withdrawing money (say $50 each) at the same time l Security problems 4 Hard to provide user access to some, but not all, data n Database systems offer solutions to all the above problems Database System Concepts - 6 th Edition 1. 4
Levels of Abstraction n Physical level: describes how a record (e. g. , customer) is stored. n Logical level: describes what data stored in database, and the relationships among the data. l Example (in PASCAL): type instructor = record ID : string; name : string; dept_name : string; salary : integer; end; n View level: application programs hide details of data types. Views can also hide information (such as an employee’s salary) for security purposes. Database System Concepts - 6 th Edition 1. 5
View of Data An architecture for a database system Naïve user Application programmer DBA Database System Concepts - 6 th Edition 1. 6
Instances and Schemas n n Schema – the logical structure of the database l Example: The database consists of information about a set of customers and accounts and the relationship between them l Analogous to type information of a variable in a program l Physical schema: database design at the physical level l Logical schema: database design at the logical level Instance – the actual content of the database at a particular point in time l n Analogous to the value of a variable Physical Data Independence – the ability to modify the physical schema without changing the logical schema l Applications depend on the logical schema l In general, the interfaces between the various levels and components should be well defined so that changes in some parts do not seriously influence others. Database System Concepts - 6 th Edition 1. 7
Data Models n A collection of tools for describing Data l Data relationships l Data semantics l Data constraints l n Relational model n Entity-Relationship data model (mainly for database design) n Object-based data models (Object-oriented and Object-relational) n Semistructured data model (XML) n Other older models: l l Network model Hierarchical model Database System Concepts - 6 th Edition 1. 8
Relational Model n Relational model (Chapter 2) n Example of tabular data in the relational model Columns Rows Database System Concepts - 6 th Edition 1. 9
A Sample Relational Database System Concepts - 6 th Edition 1. 10
Data Manipulation Language (DML) n Language for accessing and manipulating the data organized by the appropriate data model l DML also known as query language n Two classes of languages l Procedural – user specifies what data is required and how to get those data l Declarative (nonprocedural) – user specifies what data is required without specifying how to get those data n SQL is the most widely used query language Database System Concepts - 6 th Edition 1. 11
Data Definition Language (DDL) n Specification notation for defining the database schema Example: create table instructor ( ID char(5), name varchar(20), dept_name varchar(20), salary numeric(8, 2)) n DDL compiler generates a set of tables stored in a data dictionary n Data dictionary contains metadata (i. e. , data about data) l Database schema l Integrity constraints 4 Primary key – e. g. , ID uniquely identifies instructors in the instructor table 4 Referential integrity – e. g. , dept_name value in any instructor tuple must appear in department relation l Authorization Database System Concepts - 6 th Edition 1. 12
SQL n SQL: widely used non-procedural language l Example: Find the name of the instructor with ID 22222. select name from instructor where instructor. ID = ‘ 22222’ l Example: Find the instructor ID and department name of all instructors associated with a department with budget of greater than $95000. select instructor. ID, department. dept name from instructor, department where instructor. dept name= department. dept name and department. budget > 95000 n Application programs generally access databases through one of l Language extensions to allow embedded SQL l Application program interface (e. g. , ODBC/JDBC) which allow SQL queries to be sent to a database n Chapters 3, 4 and 5 Database System Concepts - 6 th Edition 1. 13
Database Design The process of designing the general structure of the database: n Logical Design – Deciding on the database schema. Database design requires that we find a “good” collection of relation schemas. l Business decision – What attributes should we record in the database? l Computer Science decision – What relation schemas should we have and how should the attributes be distributed among the various relation schemas? n Physical Design – Deciding on the physical layout of the database Database System Concepts - 6 th Edition 1. 14
Database Design? n Is there any problem with this design? Database System Concepts - 6 th Edition 1. 15
Design Approaches n Normalization Theory (Chapter 8) l Formalize what designs are bad, and test for them n Entity Relationship Model (Chapter 7) l Models an enterprise as a collection of entities and relationships 4 Entity: a “thing” or “object” in the enterprise that is distinguishable from other objects – Described by a set of attributes 4 Relationship: l an association among several entities Represented diagrammatically by an entity-relationship diagram: Database System Concepts - 6 th Edition 1. 16
Object-Relational Data Models n Relational model: flat, “atomic” values n Object Relational Data Models l Extend the relational data model by including object orientation and constructs to deal with added data types. l Allow attributes of tuples to have complex types, such as sets l Preserve relational foundations, in particular the declarative access to data, while extending modeling power. l Provide upward compatibility with existing relational languages. Database System Concepts - 6 th Edition 1. 17
XML: Extensible Markup Language n Defined by the WWW Consortium (W 3 C); originally intended as a document markup language not a database language n The ability to specify new tags, and to create nested tag structures made XML a great way to exchange data, not just documents <bank> <account_number> A-101 </account_number> <branch_name> Downtown </branch_name> <balance> 500 </balance> </account> <depositor> <account_number> A-101 </account_number> <customer_name> Johnson </customer_name> </depositor> </bank> n XML has become the basis for all new generation data interchange formats. A wide variety of tools is available for parsing, browsing and querying XML documents/data Database System Concepts - 6 th Edition 1. 18
Database System Internals Database System Concepts - 6 th Edition 1. 19
Storage Management n Storage manager is a program module that provides the interface between the low-level data stored in the database and the application programs and queries submitted to the system. n The storage manager is responsible for the following tasks: l Interaction with the file manager l Efficient storing, retrieving and updating of data n Issues: l Storage access l File organization l Indexing and hashing Database System Concepts - 6 th Edition 1. 20
Query Processing 1. Parsing and translation 2. Optimization 3. Evaluation Database System Concepts - 6 th Edition 1. 21
Query Optimization n Alternative ways of evaluating a given query l Equivalent expressions l Different algorithms for each operation n Cost difference between a good and a bad way of evaluating a query can be enormous n Need to estimate the cost of operations l Depends critically on statistical information about relations which the database must maintain l Need to estimate statistics for intermediate results to compute cost of complex expressions Database System Concepts - 6 th Edition 1. 22
Transaction Management n What if the system fails? n What if more than one user is concurrently updating the same data? n A transaction is a collection of operations that performs a single logical function in a database application n Transaction-management component ensures that the database remains in a consistent (correct) state despite system failures (e. g. , power failures and operating system crashes) and transaction failures. n Concurrency-control manager controls the interaction among the concurrent transactions, to ensure the consistency of the database. Database System Concepts - 6 th Edition 1. 23
Database Architecture The architecture of a database systems is greatly influenced by the underlying computer system on which the database is running: n Centralized n Client-server (see the next page) n Parallel (multi-processor) n Distributed Database System Concepts - 6 th Edition 1. 24
Two tier/Three tier Architecture Database System Concepts - 6 th Edition 1. 25
Database Users are differentiated by the way they expect to interact with the system n Naïve users – invoke one of the permanent application programs that have been written previously l Examples: people accessing database over the web, bank tellers, clerical staff n Sophisticated users – form requests in a database query language n Application programmers – computer professionals who write application programs. l Rapid application development (RAD) tools can help construct forms and reports with minimal programming efforts. n Specialized users – write specialized database applications that do not fit into the traditional data processing framework Database System Concepts - 6 th Edition 1. 26
Database Administrator (DBA) n Have central control of both the data and the programs that access those data. n Database administrator's duties include: l Schema definition l Storage structure and access method definition l Schema and physical organization modification l Granting user authority to access the database l Routine maintenance Ø Periodically back up the database Ø Monitoring the disk space Ø Monitoring the performance Database System Concepts - 6 th Edition 1. 27
History of Database Systems n 1950 s and early 1960 s: l Data processing using magnetic tapes for storage 4 Tapes l provided only sequential access Punched cards for input n Late 1960 s and 1970 s: l Hard disks allowed direct access to data l Network and hierarchical data models in widespread use l Ted Codd defines the relational data model 4 win the ACM Turing Award for this work 4 IBM 4 UC l Research begins System R prototype Berkeley begins Ingres prototype High-performance (for the era) transaction processing Database System Concepts - 6 th Edition 1. 28
History (cont. ) n 1980 s: Research relational prototypes evolve into commercial systems 4 SQL becomes industrial standard l Parallel and distributed database systems l Object-oriented database systems n 1990 s: l Large decision support and data-mining applications l Large multi-terabyte data warehouses l Emergence of Web commerce l n Early 2000 s: XML and XQuery standards l Automated database administration n Later 2000 s: l Giant data storage systems 4 Google Big. Table, Yahoo PNuts, Amazon, . . l Database System Concepts - 6 th Edition 1. 29
- Slides: 29