Chapter 1 Introduction Database System Concepts 5 th

Chapter 1: Introduction Database System Concepts, 5 th Ed. ©Silberschatz, Korth and Sudarshan See www. db-book. com for conditions on re-use

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 - 5 th Edition, May 23, 2005 1. 2 ©Silberschatz, Korth and Sudarshan

Database Management System (DBMS) n DBMS contains information about a particular enterprise l 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: all transactions l Airlines: reservations, schedules l Universities: registration, grades l 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 Database System Concepts - 5 th Edition, May 23, 2005 1. 3 ©Silberschatz, Korth and Sudarshan

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 l file formats, duplication of information in different files Difficulty in accessing data 4 Need to write a new program to carry out each new task 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 to add new constraints or change existing ones Database System Concepts - 5 th Edition, May 23, 2005 1. 4 ©Silberschatz, Korth and Sudarshan

Purpose of Database Systems (Cont. ) n Drawbacks of using file systems (cont. ) Atomicity of updates 4 Failures may leave database in an inconsistent state with partial updates carried out 4 Example: Transfer of funds from one account to another should either complete or not happen at all l Concurrent access by multiple users 4 Concurrent accessed needed for performance 4 Uncontrolled concurrent accesses can lead to inconsistencies – Example: Two people reading a balance and updating it 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 l Database System Concepts - 5 th Edition, May 23, 2005 1. 5 ©Silberschatz, Korth and Sudarshan

Levels of Abstraction n Physical level: describes how a record (e. g. , customer) is stored. n Logical level: describes data stored in database, and the relationships among the data. type customer = record customer_id : string; customer_name : string; customer_street : string; customer_city : 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 - 5 th Edition, May 23, 2005 1. 6 ©Silberschatz, Korth and Sudarshan

View of Data An architecture for a database system Database System Concepts - 5 th Edition, May 23, 2005 1. 7 ©Silberschatz, Korth and Sudarshan

Instances and Schemas n Similar to types and variables in programming languages n Schema – the logical structure of the database l l Analogous to type information of a variable in a program l Physical schema: database design at the physical level l n Example: The database consists of information about a set of customers and accounts and the relationship between them) 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 - 5 th Edition, May 23, 2005 1. 8 ©Silberschatz, Korth and Sudarshan

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 - 5 th Edition, May 23, 2005 1. 9 ©Silberschatz, Korth and Sudarshan

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 - 5 th Edition, May 23, 2005 1. 10 ©Silberschatz, Korth and Sudarshan

Data Definition Language (DDL) n Specification notation for defining the database schema Example: create table account ( account-number char(10), balance integer) 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 Data storage and definition language 4 Specifies the storage structure and access methods used l Integrity constraints 4 Domain constraints 4 Referential integrity (references constraint in SQL) 4 Assertions l Authorization Database System Concepts - 5 th Edition, May 23, 2005 1. 11 ©Silberschatz, Korth and Sudarshan

Relational Model Attributes n Example of tabular data in the relational model Database System Concepts - 5 th Edition, May 23, 2005 1. 12 ©Silberschatz, Korth and Sudarshan

A Sample Relational Database System Concepts - 5 th Edition, May 23, 2005 1. 13 ©Silberschatz, Korth and Sudarshan

SQL n SQL: widely used non-procedural language l Example: Find the name of the customer with customer-id 192 -83 -7465 select customer_name from customer where customer_id = ‘ 192 -83 -7465’ l Example: Find the balances of all accounts held by the customer with customer-id 192 -83 -7465 select account. balance from depositor, account where depositor. customer_id = ‘ 192 -83 -7465’ and depositor. account_number = account_number 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 Database System Concepts - 5 th Edition, May 23, 2005 1. 14 ©Silberschatz, Korth and Sudarshan

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 - 5 th Edition, May 23, 2005 1. 15 ©Silberschatz, Korth and Sudarshan

The Entity-Relationship Model n Models an enterprise as a collection of entities and relationships l Entity: a “thing” or “object” in the enterprise that is distinguishable from other objects 4 Described l by a set of attributes Relationship: an association among several entities n Represented diagrammatically by an entity-relationship diagram: Database System Concepts - 5 th Edition, May 23, 2005 1. 16 ©Silberschatz, Korth and Sudarshan

Object-Relational Data Models n Extend the relational data model by including object orientation and constructs to deal with added data types. n Allow attributes of tuples to have complex types, including non-atomic values such as nested relations. n Preserve relational foundations, in particular the declarative access to data, while extending modeling power. n Provide upward compatibility with existing relational languages. Database System Concepts - 5 th Edition, May 23, 2005 1. 17 ©Silberschatz, Korth and Sudarshan

XML: Extensible Markup Language n Defined by the WWW Consortium (W 3 C) n 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 n XML has become the basis for all new generation data interchange formats. n A wide variety of tools is available for parsing, browsing and querying XML documents/data Database System Concepts - 5 th Edition, May 23, 2005 1. 18 ©Silberschatz, Korth and Sudarshan

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 to 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 - 5 th Edition, May 23, 2005 1. 19 ©Silberschatz, Korth and Sudarshan

Query Processing 1. Parsing and translation 2. Optimization 3. Evaluation Database System Concepts - 5 th Edition, May 23, 2005 1. 20 ©Silberschatz, Korth and Sudarshan

Query Processing (Cont. ) 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 - 5 th Edition, May 23, 2005 1. 21 ©Silberschatz, Korth and Sudarshan

Transaction Management 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 - 5 th Edition, May 23, 2005 1. 22 ©Silberschatz, Korth and Sudarshan

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 n Parallel (multi-processor) n Distributed Database System Concepts - 5 th Edition, May 23, 2005 1. 23 ©Silberschatz, Korth and Sudarshan

Database Users are differentiated by the way they expect to interact with the system n Application programmers – interact with system through DML calls n Sophisticated users – form requests in a database query language n Specialized users – write specialized database applications that do not fit into the traditional data processing framework 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 Database System Concepts - 5 th Edition, May 23, 2005 1. 24 ©Silberschatz, Korth and Sudarshan

Database Administrator n Coordinates all the activities of the database system; the database administrator has a good understanding of the enterprise’s information resources and needs. n Database administrator's duties include: l Schema definition l l l Storage structure and access method definition Schema and physical organization modification Granting user authority to access the database Specifying integrity constraints Acting as liaison with users Monitoring performance and responding to changes in requirements Database System Concepts - 5 th Edition, May 23, 2005 1. 25 ©Silberschatz, Korth and Sudarshan

Overall System Structure Database System Concepts - 5 th Edition, May 23, 2005 1.

History of Database Systems n 1950 s and early 1960 s: l Data processing using magnetic tapes for storage 4 Tapes l provide only sequential access Punched cards for input n Late 1960 s and 1970 s: l Hard disks allow direct access to data l Network and hierarchical data models in widespread use l Ted Codd defines the relational data model 4 Would 4 IBM 4 UC l win the ACM Turing Award for this work Research begins System R prototype Berkeley begins Ingres prototype High-performance (for the era) transaction processing Database System Concepts - 5 th Edition, May 23, 2005 1. 27 ©Silberschatz, Korth and Sudarshan

History (cont. ) n 1980 s: l 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 n 2000 s: l XML and XQuery standards l Automated database administration Database System Concepts - 5 th Edition, May 23, 2005 1. 28 ©Silberschatz, Korth and Sudarshan

End of Chapter 1 Database System Concepts, 5 th Ed. ©Silberschatz, Korth and Sudarshan See www. db-book. com for conditions on re-use