Database System Concepts · •Databases touch all aspects of our lives ... –Entity Relationship Model (Chapter 7) •Models an enterprise as a collection of entities and relationships

Database System Concepts

Chapter 1: Introduction 9/7/2020

Slides• Slides are modified based on the resources on

the textbook’s official website – http://www.db-book.com/

– https://www.db-book.com/db7/index.html

– https://www.db-book.com/db7/university-lab-dir/sqljs.html

• 网易云课堂– https://mooc.study.163.com/course/1000031000#/info

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http://www.db-book.com/https://www.db-book.com/db7/index.htmlhttps://www.db-book.com/db7/university-lab-dir/sqljs.htmlhttps://mooc.study.163.com/course/1000031000#/info

Outline

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• The Need for Databases

• View of Data

• Data Models

• Database Languages

• Database Design

• Storage Manager

• Query Processing

• Transaction Manager

• History of Database Systems

Outline

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• View of Data

• Data Models


• Database Design

• Storage Manager




Database Management System (DBMS)

• Databases can be very large.

– define structure for information storage

– provide mechanisms for information manipulation(操作)

– ensure information safety

• 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

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Database Management System (DBMS) (Cont.)

• Databases touch all aspects of our lives (applications):– Banking: transactions– Airlines: reservations, schedules– Universities: registration, grades– Sales: customers, products, purchases– Online retailers: order tracking, customized

recommendations– Manufacturing (制造业) : production, inventory,

orders, supply chain– Human resources: employee records, salaries, tax

deductions

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DB Table Example

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这样的设计好么？

• 学生分数表设计

Reference:https://mooc.study.163.com/course/ZJU-1000031000#/info

DB Table Example

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Course

• 另一种设计Students

Enrolled

这样的设计好么？为什么？

Location


DB Table Example

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结构不稳定• 学生分数表设计


信息冗余

University Database Example

• Suppose a university organization, keep information about – instructor, students, departments, and course offering

• To manipulate the information, a number of application program examples– add new students, instructors, and courses– register students for courses, and generate class rosters（班级名单）

– assign grades to students, compute grade point averages (GPA) and generate transcripts (成绩单)

• In the early days, database applications were built directly on top of file systems 10

Drawbacks of using file systems to store data

• Data redundancy(冗余) and inconsistency(不一致)

– Multiple file formats, duplication of information in different files

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ExampleStudent, double major: Math, Music

• records in the Math Dept.• records in the Music Dept.

Duplication info in two departmentsData inconsistency if changes make in one place


• Difficulty in accessing data

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

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ExampleQuery 1:

to generate the list of all the students

Query 2:to generate the list of students in Math Dept.

Do not allow needed data to be retrieved in a convenient and efficient manner


• Data isolation (隔离性)– Multiple files and formats

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ExampleData are scattered in various files, and files may bein different formats

Difficult to write new applications to retrieve theappropriate data


• Integrity(完整性) 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

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ExampleSuppose the university maintains an account for each dept.,and records the balance amount in each account >0

Write code in various programs to enforce these constraintsNew constraints added, difficult to change the program to enforce them


• Atomicity(原子性) of updates

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

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ExampleConsider a program to transfer $500 from the account balance of Dept. A to Dept. B. If a system failure occursduring the program execution.

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

Drawbacks of using file systems to store data (Cont.)

• Concurrent(并发) access by multiple users

– Concurrent access needed for performance

– Uncontrolled concurrent accesses can lead to inconsistencies

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

Drawbacks of using file systems to store data (Cont.)

• Security problems

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

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ExampleStudents can only view their own information

In file-processing system, application programs are addedto the system in an ad hoc (特设) manner, enforcing such security constraints is difficult!


• Data redundancy and inconsistency

• Difficulty in accessing data

• Data isolation

• Integrity problems

• Atomicity of updates

• Concurrent access by multiple users

• Security problems

21Database systems offer solutions to all the above problems

Outline

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• View of Data

• Data Models


• Database Design

• Storage Manager




Levels of Abstraction

• Physical level: describes how a record (e.g., instructor) is stored.

• Logical level: describes data stored in database, and the relationships among the data.

type instructor = recordID : string; name : string;dept_name : string;salary : integer;end;

• View level: application programs hide details of data types. Views can also hide information (such as an employee’s salary) for security purposes.

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View of Data

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An architecture for a database system

Instances and Schemas

• Schema (模式)– the overall logical/physical structure of the database– analogous to variable declarations (with type information) in a

program

• Instance (实例)– the actual content of the database at a particular point in time – analogous to the values of a variable at a point in time

• Physical Data Independence– applications do NOT depend on the physical schema– the ability to modify the physical schema without changing 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.

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Outline

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• View of Data

• Data Models


• Database Design

• Storage Manager




Data Models• A collection of tools for describing

– Data – Data relationships– Data semantics (语义)– Data constraints

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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

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Relational Model• All the data is stored in various tables.

• Example of tabular data in the relational model

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Columns

Rows

A Sample Relational Database

instructor

department






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The Entity-Relationship Model

• Models an enterprise as a collection of entities and relationships

– Entity: a “thing” or “object” in the enterprise that is distinguishable from other objects• Described by a set of attributes

– Relationship: an association among several entities

• Represented diagrammatically by an entity-relationship diagram:






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Object-Relational Data Models

• Relational model: flat, “atomic” values• Object Relational Data Models

– extend the relational data model by including object orientation and constructs to deal with added data types;

– allow attributes of tuples to have complex types, including non-atomic values such as nested relations;

– preserve relational foundations, in particular the declarative access to data, while extending modeling power;

– provide upward compatibility with existing relational languages.

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XML: Extensible Markup Language

• Individual data items of the same type may have different setof attributes

• Defined by the WWW Consortium (W3C)

• Originally intended as a document markup language not adatabase language

• The ability to specify new tags, and to create nested tagstructures made XML a great way to exchange data, not justdocuments

• XML has become the basis for all new generation datainterchange formats.

• A wide variety of tools is available for parsing, browsing andquerying XML documents/data

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• Semi-structured data model (XML)


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Outline

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• View of Data

• Data Models


• Database Design

• Storage Manager




Data Definition Language (DDL)

• Specification notation for defining the database schemaExample: create table instructor (

ID char(5),name varchar(20),dept_name varchar(20),salary numeric(8,2))

• DDL compiler generates a set of table templates stored in a data dictionary

• Data dictionary contains metadata (i.e., data about data)– Database schema

– Integrity constraints

• Primary key (ID uniquely identifies instructors)

– Authorization

• Who can access what40

Data Manipulation Language (DML)

• Language for accessing and manipulating the dataorganized by the appropriate data model

– DML also known as query language

• Two classes of languages

– Pure – used for proving properties about computationalpower and for optimization• Relational Algebra

• Tuple relational calculus

• Domain relational calculus

– Commercial – used in commercial systems• SQL is the most widely used commercial language

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SQL

• The most widely used commercial language

• SQL is NOT a Turing machine equivalent language

• To be able to compute complex functions SQL isusually embedded in some higher-level language

• Application programs generally access databasesthrough 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

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Outline

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• View of Data

• Data Models


• Database Design

• Storage Manager




Database Design

• The process of designing the general structure ofthe database.– Logical Design, deciding on the database schema.

Database design requires that we find a “good”collection of relation schemas.• Business decision: what attributes should we record in the

database?

• Computer Science decision: what relation schemas shouldwe have and how should the attributes be distributedamong the various relation schemas?

– Physical Design, deciding on the physical layout of thedatabase

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Database Design (Cont.)

• Is there any problem with this relation?

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Design Approaches

• Need to come up with a methodology to ensurethat each of the relations in the database is“good”

• Two ways of doing so:– Entity Relationship Model (Chapter 7)

• Models an enterprise as a collection of entities andrelationships

• Represented diagrammatically by an entity-relationshipdiagram:

– Normalization Theory (Chapter 8)• Formalize what designs are bad, and test for them

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Database System Internals

Database Users and Administrators

Database

Database Architecture

• The architecture of a database systems is greatlyinfluenced by the underlying computer system onwhich the database is running:

– Centralized

– Client-server

– Parallel (multi-processor)

– Distributed

Outline

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• View of Data

• Data Models


• Database Design

• Storage Manager




Storage Management

• Storage manager is a program module that providesthe interface between the low-level data stored in thedatabase and the application programs and queriessubmitted to the system.

• The storage manager is responsible to the followingtasks:– Interaction with the OS file manager– Efficient storing, retrieving and updating of data

• Issues:– Storage access– File organization– Indexing and hashing

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Query Processing

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1. Parsing and translation2. Optimization3. Evaluation

Query Processing (Cont.)

• Alternative ways of evaluating a given query

– Equivalent expressions

– Different algorithms for each operation

• Cost difference between a good and a bad way

of evaluating a query can be enormous

• Need to estimate the cost of operations

– depends critically on statistical information about

relations which the database must maintain

– need to estimate statistics for intermediate results

to compute cost of complex expressions

Transaction Management• What if the system fails?

• What if more than one user is concurrently updatingthe same data?

• A transaction is a collection of operations thatperforms a single logical function in a databaseapplication

• Transaction-management component ensures thatthe database remains in a consistent (correct) statedespite system failures (e.g., power failures andoperating system crashes) and transaction failures.

• Concurrency-control manager controls theinteraction among the concurrent transactions, toensure the consistency of the database.

Outline

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• View of Data

• Data Models


• Database Design

• Storage Manager




History of Database Systems

• History of Database [video]• 1950s and early 1960s:

– Data processing using magnetic tapes for storage• Tapes provided only sequential access

– Punched cards for input

• Late 1960s and 1970s:– Hard disks allowed 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

https://www.youtube.com/watch?v=KG-mqHoXOXY

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

• Early 2000s:– XML and XQuery standards– Automated database administration

• Later 2000s:– Giant data storage systems

• Google BigTable, Yahoo PNuts, Amazon, ..

Thank you!

Q&A

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Database System Concepts · •Databases touch all aspects of our lives ... –Entity Relationship Model (Chapter 7) •Models an enterprise as a collection of entities and relationships