Database Systems

Database– an integrated collection of related data– Related data, e.g.: Information stored in an University

Students, Courses, Faculty, Students taking courses, Faculty teaching courses, ....

– integrated:all data is stored in a uniform way on secondary storage

Database Management System– a collection of programs that is used to create, maintain and manipulate data in the database

Database System– DB + DBMS + Application Programs

Database System Organization: A Simplified View

Application Programs

Query and transaction Processing

Management of Stored Data

Meta-data Database

DBS

DBMS

Users

Databases vs File Systems

What is wrong with a File System?– Data Integration and Data Sharing

Features of DBMS that cannot be provided with a file system

– Data Consistency– Controlled Redundancy– Program-Data Independence– Integrity Enforcement– Concurrency Control– Backup and Recovery– Security and Privacy– Multiple views of Data

Additional Advantages Performance Expandability/Flexibility Reduced Applicaiton Development Time Enforcement of Standards Economies of Scale

The Price You Pay !! High initalcost High overhead Not special purpose

When is DBMS Inappropriate?

Database is small and has simple structure applications are simple and special-purpose applications with real-time requirements concurrent, multi-user access to data is not needed

The Three Levels of Abstraction

Internal Level– describes the physical storage structure of the DB

Conceptual Level– describes the structure of the whole DB– hides storage and implementationdetails

External Level– point of view of users

Logical and Physical Data Independence

Data Modeling / Database Design Database Design

– is the activity of specifying the schema of a database in a given data model

Database Schema– is the structure of a database that

captures data types, relationships, constriants on the data is independent of any application programchanges infrequently

Database instance or state – the actual data in the database at a given time

Data Model– a set of primitives for defining the structure of a DB– a set of operations for specifying the retrievals and updates on a DB

– relational, hierarchical, network, object-oriented, .....

Relational Model (Codd 1970) The most popular implementation model

– simplest, has the most uniform data structures,has a formal mathematical model, powerful query languages (relational algebra), existence of 4th generation languages

– but, not suitable for some applications Everything is represented by relations

– Formally: Given sets D1, D2, ....Dn (not necessarily

distinct), a relation R D1 X D2 X ...X Dn

– Di 's are the domains and n is the arity (degree) of R– elements of R are called tuples– number of tuples in R is the cardinality of R

relational data model helps to view a relation as a table

– each row represents a tuple (record)– each column represnts an attribute (field)

Observe the following properties:– no two rows are identical– the ordering of tuples is unimportant

– the ordering of columns is important

Relational Model (continued)

Part # PName Color Weight

P1 Nut Red 12

P2 Bolt Blue 17

P3 Screw Green 16

PART

Relation Schema A relation scheme R specifies

– the attribute names Ai of R

– the domain Di (datatype + format) for each Aidatatype is a set of atomic data values: no attribute is set-valued (1st Normal Form or, 1-NF) no attribute is composite

– format is the specification of the representationof a data values

A collection of relation schema used to represent the information in the database is the database scheme

A relation instance r of R (denoted r(R)) is the set of tuples that compose the relation at a given intance, i.e. the current values.

cardinality |r(PARTS)| = 3, the arity |PARTS| = 4 In general, |R| > 0, |r(R)| 0

Keys

Let R be a realtion schema and K R K is a superkey of R if it can uniquly identify any tuple in any

r(R). There are no tuples t and t' such that t[K] = t'[K} K is a candidate key if K is a minimal superkey. There is no

K' K such that K' is also a superkey of r(R) A primary key is one of the candidate keys, remaining

candidate keys are alternate keysE.g.: CLASS (Course#, Prof, Sched, Room)Identify superkeys, candidate keys

Key is a property of a relation schema but is not of a relation

Relational Database Schema

A database schema is a set of relation schemas and a set of integrity constraints

Integrity constraints– structural

key constraints: uniqueness of keysentity integrity constraint: no primary key value can be nullreferential integrity constraint

– semantic

Referential Integrity Constraints In the relational model, the only way an entity can reference

another entity is through the value of the primary key of the second entity

A foreign key (FK) is a set of one or more attributes of a relation R1

that forms a primary key (PK) of another relation R2 This means

– the attributes in FK have the same domain as the primary key attributes of R2

– the value of FK in any tuple t1 of r(R1) is either null or matches with a value of

PK for some tuple t2 in r(R2), i.e., t1[FK] = t2[PK]EMP SSN EName DNO

DEPT DNO DName Mgr

Each employee must belong to some department

we say attributes FK of R1 reference or refer to the relation R2 Referential integrity constraints can be defined for the same

relation, i.e., tuples may refer to another tuple in the same relation

Referential Integrity Constraints (continued)

EMP SSN EName DNO SUPERSSN

Query languages allow manipulation and retrieval of data from a database

Relational model supports simple, powerful query languages– strong formal foundation based on logic– allows for optimization

Two mathematical languages form the basis for rel languages (e.g., SQL) and for implementation

– Relational Algebra: More operational, useful for representing execution plans

– Relational Calculus: Lets users describe what they want, rather than how to compute it (non-operational, declarative)

Basic operations:– selection, projection, cross-product, set-difference, union,

intersection, join, division

Relational Query Languages

SQL– SQL (Structured Query Language) is the query language for the

System R developed at IBM San Jose [Astraham, Gray, Lindsay, Selinger ..]

– SQL is now the query language for IBM's DB2 and the de-facto standard on most commercial RDBMS

– SQL is a comprehensive language providing statements for data definition, query and update. Hence it is both DDL and DML

– SQL allows to create views, it can be embedded in a general-purpose programming language (C or PASCAL)

– SQL has one basic statement for retrieving data from the database: the SELECT statement

SELECT <attribute list>FROM <table list>WHERE <condition>

– Standards: SQL or SQL1 (ANSI 1986) SQL2 or SQL-92 (ANSI 1992) SQL3 underway: extends SQL with OO and other concepts

SQL Data Types

Numeric– Integers of various ranges: INTEGER (or INT), SMALLINT– Real numbers of various precision: FLOAT, REAL, DOUBLE

PRECISION– Formatted numbers: DECIMAL(i,j) or DEC(i,j) or NUMERIC(i,j)

Character Strings– Fixed length n: CHAR(n) or CHARACTER(n)– Variable length of maximum n: VARCHAR(n) or CHAR

VARYING(n) (default n =1) Bit strings

– Fixed length n: BIT(n)– Varying length of maximum n: VARBIT(n) or BIT VARYING(n)

SQL Data Types (continued)

Date & Time [SQL2]– DATE (10 positions): YYYY-MM-DD– TIME (8 positions): HH:MM:SS– TIME(i) defines i decimal fractions of seconds (8+1+i positions): HH:MM:SS:ddd...d– TIME WITH TIME ZONE includes the displacement from

standard universal time zone [+13:00 to -12:59] (6 additional positions): HH:MM:SS+/-HH:MM

– TIMESTAMP:date, time with 6 fractions of seconds and optional time zone

– INTERVAL: Year/Month or Day/TIME

DDL

DDL is used to define the (schema of) database– to create a database schema– to create a domain – to create, drop. alter a table– to create, remove an index [defunct in SQL2]– to create or drop a view– to define integrity constraints– to define access privileges to users (Oracle: CONNECT,

RESOURCE, DBA)– to GRANT or REVOKE privileges ON/TO object/user

SQL2 supports multiple schemas– CREATE SCHEMA name AUTHORIZATION user;– CREATE SCHEMA EMPLOYEE AUTHORIZATION atluri;

Create Domain

CREATE DOMAIN name_dom AS VARCHAR(30); CREATE DOMAIN project_dom AS CHAR(20); CREATE DOMAIN dept_dom AS VARCHAR(20)

DEFAULT 'none'; CREATE DOMAIN city_dom CHAR(20) DEFAULT

NULL; CREATE DOMAIN hour_dom FLOAT DEFAULT

0; CREATE DOMAIN gender_dom CHAR(1) CHECK (VALUE IN ('F', 'f', 'M', 'm'));

SQL Schema EMP(Name,SSN,DNO,BirthPlace) DEPT(DName,DNO,MGRSSN) PROJECT(PName,PNO,PLocation,DNum) WORKSON(ESSN,PNO,Hours)

CREATE SCHEMA 'COMPANY';

CREATE TABLE EMP( EName name_domNOT NULL, SSN CHAR(9) NOT NULL, DNO INTEGER NOT NULL, BirthPlace city_dom, PRIMARY KEY(SSN), FOREIGN KEY (DNO) REFERENCES DEPT (DNO));

Constraints Constraints on attributes

NOT NULL constraintDEFAULT value allows the specification of default value (without the default

clause, the default value is NULL)PRIMARY KEY (attribute-list)UNIQUE (attribute list) allows the specification of alternative keyFOREIGN KEY (key) REFERENCES table (key)

Enforcement of Time Constraints ImmediateDeferrable (until commit time)

Actions if a referential integrity constraint is violated (referential triggered actions):SET NULLCASCADE (propagate action)SET DEFAULT)

Qualifying actions by the triggering condition: ON DELETE and ON UPDATE FOREIGN KEY (DNO) REFERENCES DEPT (DNO) ON DELETE SET DEFAULT ON UPDATE CASCADE

Naming of the ConstraintsKeyword CONSTRAINT may be used to name a constraintsHelpful in modifying or dropping the constraint

CREATE TABLE EMP( EName name_dom NOT NULL, SSN CHAR(9) NOT NULL, DNO INTEGER NOT NULL, BirthPlace city_dom, CONSTRAINT Emp_PK PRIMARY KEY(SSN), CONSTRAINT Emp_FK FOREIGN KEY (DNO) REFERENCES DEPT (DNO));

System Catalog (Dictionary)

Dictionary stores a set of tables that describe the database:

– Base Relations (tables) possible attributes:table-name, creator, #of-tuples, tuple-length, #of- attributes, ..

– Attributes of Relations (columns)possible attributes: table-name, attribute-name, format, order, key. ,,

– Indexespossible attributes: table-name, index-name, key-attribute, ..

– Authorization– Integrity– In Oracle, the dictionary is made up of tablespaces (one or more physical files): SYSTEM, USERS, TEMP, APPLICATIONS

DROP command can be used to remove– a schema: DROP SCHEMA Company CASCADE; DROP SCHEMA Company RESTRICT

CASCADE option removes everything: tuples, tables, domains, ...RESTRICT option removes the schema if it has no elements in it

– a table: DROP TABLE EMP CASCADE; DROP SCHEMA EMP RESTRICT

CASCADE option removes the table and all references to itRESTRICT option removes the table if it is not referenced

DROP Command

ALTER CommandThe ALTER allows to:

– alter the domain of an attribute ALTER TABLE Student – ALTER GPA NUMBER(4,2);– set or drop default value of an attribute ALTER TABLE Student ALTER GPA DROP DEFAULT; ALTER TABLE Student ALTER GPA SET DEFAULT 0.00;– add a new attribute to a relation ALTER TABLE Student ALTER Admission DATE;– drop an attribute (not in SQL1) ALTER TABLE Student DROP GPA [CASCADE/RESTRICT];

The Select Statement– The general form of a SELECT statement:

SELECT <attribute list>FROM <table list>WHERE <condition>GROUP BY <attribute list>HAVING <condition>ORDER BY <attribute,{ASC/DESC} pair>

Relational Operators in SQL– Projection:

SELECT A,BFROM R

– Selection: SELECT *FROM RWHERE F

– Product of two tables: A X BSELECT R., S.FROM R, S

Query: List the names of all employees that work in CS

SELECT NameFROM EMPWHERE Dept = CS

Renaming of attributes:

SELECT Name AS CSNameFROM EMPWHERE Dept = CS

SELECT DISTINCT BirthPlaceFROM EMP(UNIQUE is not valid any more in SQL2)

More Queries

Give the number of all employees in the CS Department

SELECT COUNT()FROM EMPWHERE Dept = CS

Give the number of employees in each departmentSELECT Dept, COUNT()FROM EMPGROUPBY Dept

Give the names of the departments that have more than 50 employees. Also list the number of employees in those departments

SELECT Dept, COUNT()FROM EMPGROUPBYDeptHAVING COUNT() > 50

More SQL Built-in FunctionsSUM, AVG,MAX,MIN (List the employee names who make more than the

average salary of all employees)

Some More ..