Chapter 1: Introduction

Chapter 1: Introduction

Database System Concepts, 6th Ed. ©Silberschatz, Korth and Sudarshan See www.db-book.com for conditions on re-use

Data, Database, and DBMS n Data l

[Elmasri and Navathe. Fundamentals of Database Systems]

Known facts that can be recorded and that have implicit meaning

e.g.) names, phone numbers, addresses, … n Database l

Collection of interrelated data

n Database management system (DBMS) l

Database + set of programs to access those data

l

An environment that is both convenient and efficient to use

Database System Concepts - 6th Edition

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©Silberschatz, Korth and Sudarshan

Database System Applications n Databases touch all aspects of our lives

n Database Applications: l

Banking: 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


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Drawbacks of Using File Systems to Store Data n Data redundancy and inconsistency l

Multiple file formats, duplication of information in different files

n Difficulty in accessing data l

Need to write a new program to carry out each new task

n Data isolation l

Data are scattered in multiple files and formats

n Integrity problems l

Integrity constraints (e.g., account balance > 0) become “buried” in program code rather than being stated explicitly

l

Hard to add new constraints or change existing ones


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Drawbacks of Using File Systems to Store Data (Cont.) n Atomicity of updates l

Failures may leave database in an inconsistent state with partial updates carried out

l

Example: Transfer of funds from one account to another should either complete or not happen at all

n Concurrent access by multiple users l

Concurrent access needed for performance

l

Uncontrolled concurrent accesses can lead to inconsistencies 4

Example: Two people reading a balance (say 100) and updating it by withdrawing money (say 50 each) at the same time

n Security problems l

Hard to provide user access to some, but not all, data

Database systems offer solutions to all the above problems


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Levels of Abstraction n View level: application programs hide details of data types. Views can also

hide information (such as an employee’s salary) for security purposes. n Logical level: describes data stored in database, and the relationships among

the data type instructor = record ID : string; name : string; dept_name : string; salary : integer; end; n Physical level:

describes how a record is stored


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Instances and Schemas n Similar to types and variables in programming languages

n Schema – the logical structure of the database l

Analogous to type information of a variable in a program

l

Example: The database consists of information about a set of customers and accounts and the relationship between them

n Instance – the actual content of the database at a particular point in time l

Analogous to the value of a variable


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Physical Data Independence n Schemas according to the level of abstraction l

Logical schema: database design at the logical level

l

Physical schema: database design at the physical level

n 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.


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Data Models n A collection of tools for describing l l l l

Data Data relationships Data semantics Data constraints

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


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Relational Model n Relational model (Chapter 2) n Example of tabular data in the relational model

Columns

Rows


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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

l

Described by a set of attributes

Relationship: an association among several entities

n Represented diagrammatically by an entity-relationship diagram:


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Object-Based Data Model n Supports complex types and inheritance


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XML Data Model n Supports extensible tags and nested structures

Comp. Sci. Comp. Sci. Taylor Taylor 100000 100000 CS-101 CS-101 Intro. to Computer Science Intro. to Computer Science Comp. Sci Comp. Sci 4 4


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Data Definition Language (DDL) n Language for defining the database schema

n DDL compiler generates a set of table templates stored in a

data dictionary

n Data dictionary contains metadata (i.e., data about data) l

Database schema

l

Integrity constraints 4

Primary key (ID uniquely identifies instructors)

4

Referential integrity (references constraint in SQL) – e.g. dept_name value in any instructor tuple must appear in department relation

l

Authorization


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Data Manipulation Language (DML) n Language for accessing and manipulating the data organized by the appropriate

data model l

Retrieval, insertion, deletion, modification

n Query language: part of DML that requests data retrieval l

Commonly used as a synonym for DML

n Two classes of languages l

Procedural – user specifies what data is required and how to get those data

l

Declarative (nonprocedural) – user specifies only what data is required, without specifying how to get those data

n SQL is the most widely used query language


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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 ID and building of instructors in the Physics dept. select instructor.ID, department.building from instructor, department where instructor.dept name = “physics”

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


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Query Processing 1. Parsing and translation 2. Optimization 3. Evaluation


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Storage Management n DBMS must effectively and efficiently manage storage (disk) space

n Storage manager l

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

l

Efficient storing, retrieving and updating of data

n Issues: l

Storage access

l

File organization

l

Indexing and hashing


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Transaction Management n What if the system fails? n What if more than one user is concurrently updating the same data?

n Transaction – 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


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Database Users 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


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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

Storage structure and access method definition

l

Schema and physical organization modification

l

Granting user authority to access the database

l

Specifying integrity constraints

l

Acting as liaison with users

l

Monitoring performance and responding to changes in requirements


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Overall Database System Structure


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Application Architecture

n Two-tier architecture: application programs communicate with DBMS using

API standards (like ODBC/JDBC) n Three-tier architecture: “middleware” (like WAS – Web application server) is

used for accessing data Database System Concepts - 6th Edition

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History of Database Systems n 1950s and early 1960s: l

Data processing using magnetic tapes for storage 4

l

Tapes provided only sequential access

Punched cards for input

n Late 1960s and 1970s: l

Hard disks allowed direct access to data

l

Network and hierarchical data models in widespread use

l

E. F. “Ted” Codd defines the relational data model

l

4

Would win the ACM Turing Award for this work

4

IBM Research begins System R prototype

4

UC Berkeley begins Ingres prototype

High-performance (for the era) transaction processing


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History of Database Systems (Cont.) n 1980s: l

l

Research relational prototypes evolve into commercial systems 4 SQL becomes industrial standard Parallel and distributed database systems

Object-oriented database systems n 1990s: l

l l

Large decision support and data-mining applications Large multi-terabyte data warehouses

Emergence of Web commerce n Early 2000s: l

l l

XML and XQuery standards Automated database administration

n Later 2000s: l

Giant data storage systems 4

Google BigTable, Yahoo PNuts, Amazon, ..


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End of Chapter 1


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