Database Systems Introduction PDF

Chapter 1: Introduction Purpose of Database Systems View of Data Database Languages Relational Databases Database Users and Administrators History of Database Systems  Database Management System (DBMS) DBMS contains information about a particular enterprise Collectio...

Chapter 1: Introduction Purpose of Database Systems View of Data Database Languages Relational Databases Database Users and Administrators History of Database Systems  Database Management System (DBMS) DBMS contains information about a particular enterprise Collection of interrelated data Set of programs to access the data An environment that is both convenient and efficient to use Database Applications: Banking: all transactions Airlines: reservations, schedules Universities: registration, grades Sales: customers, products, purchases Online retailers: order tracking, customized recommendations Manufacturing: production, inventory, orders, supply chain Human resources: employee records, salaries, tax deductions Databases touch all aspects of our lives Purpose of Database Systems In the early days, database applications were built directly on top of file systems Drawbacks of using file systems to store data: Data redundancy and inconsistency Multiple file formats, duplication of information in different files Difficulty in accessing data Need to write a new program to carry out each new task Data isolation — multiple files and formats Integrity problems Integrity constraints (e.g. account balance > 0) become “buried” in program code rather than being stated explicitly Hard to add new constraints or change existing ones Purpose of Database Systems (Cont.) Drawbacks of using file systems (cont.) Atomicity of updates Failures may leave database in an inconsistent state with partial updates carried out Example: Transfer of funds from one account to another should either complete or not happen at all Concurrent access by multiple users Concurrent accessed needed for performance Uncontrolled concurrent accesses can lead to inconsistencies Example: Two people reading a balance and updating it at the same time Security problems Hard to provide user access to some, but not all, data Database systems offer solutions to all the above problems Levels of Abstraction Physical level: describes how a record (e.g., customer) is stored. 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 : string; end; View level: application programs hide details of data types. Views can also hide information (such as an employee’s salary) for security purposes. View of Data An architecture for a database system Instances and Schemas Similar to types and variables in programming languages Schema – the logical structure of the database Example: The database consists of information about a set of customers and accounts and the relationship between them) Analogous to type information of a variable in a program Physical schema: database design at the physical level Logical schema: database design at the logical level Instance – the actual content of the database at a particular point in time Analogous to the value of a variable Physical Data Independence – the ability to modify the physical schema without changing the logical schema Applications depend on the logical schema 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. Data Models A collection of tools for describing Data Data relationships Data semantics Data constraints There are a number of different data models. Relational model Entity-Relationship data model (mainly for database design) Object-based data models (Object-oriented and Object-relational) Semistructured data model (XML) Other older models: Network model Hierarchical model Data Manipulation Language (DML) Language for accessing and manipulating the data organized by the appropriate data model DML also known as query language Two classes of languages Procedural – user specifies what data is required and how to get those data Declarative (nonprocedural) – user specifies what data is required without specifying how to get those data SQL is the most widely used query language Data Definition Language (DDL) Specification notation for defining the database schema Example: create table account ( account-number char(10), balance integer) DDL compiler generates a set of tables stored in a data dictionary Data dictionary contains metadata (i.e., data about data) Database schema Data storage and definition language Specifies the storage structure and access methods used Integrity constraints Domain constraints Referential integrity (references constraint in SQL) Assertions Authorization Relational Model Example of tabular data in the relational model Attributes A Sample Relational Database SQL SQL: widely used non-procedural language Example: Find the name of the customer with customer-id 192-83-7465 select customer.customer_name from customer where customer.customer_id = ‘192-83-7465’ 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.account_number Application programs generally access databases through one of Language extensions to allow embedded SQL Application program interface (e.g., ODBC/JDBC) which allow SQL queries to be sent to a database ODBC is a standard Microsoft Windows® interface that enables communication between database management systems and applications typically written in C or C++. JDBC is a standard interface that enables communication between database management systems and applications written in Oracle Java. Database Users Users are differentiated by the way they expect to interact with the system Application programmers – interact with system through DML calls Sophisticated users – form requests in a database query language Specialized users – write specialized database applications that do not fit into the traditional data processing framework Naive users – invoke one of the permanent application programs that have been written previously Examples, people accessing database over the web, bank tellers, clerical staff Database Administrator Coordinates all the activities of the database system; the database administrator has a good understanding of the enterprise’s information resources and needs. Database administrator's duties include: Schema definition 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 History of Database Systems 1950s and early 1960s: Data processing using magnetic tapes for storage Tapes provide only sequential access Punched cards for input Late 1960s and 1970s: Hard disks allow direct access to data Network and hierarchical data models in widespread use Ted Codd defines the relational data model Would win the ACM Turing Award for this work IBM Research begins System R prototype UC Berkeley begins Ingres prototype High-performance (for the era) transaction processing History (cont.) 1980s: Research relational prototypes evolve into commercial systems SQL becomes industrial standard Parallel and distributed database systems Object-oriented database systems 1990s: Large decision support and data-mining applications Large multi-terabyte data warehouses Emergence of Web commerce 2000s: XML and XQuery standards Automated database administration End of Chapter 1