Garcia-Molina, Ullman, and Widom

Database SystemsChapter 2

Data Models

• A data model is a notation for describing the structure of the data in a database , – along with the constraints on that data.

• a day of the week is an integer between 1 and 7

• The data model also normally provides a notation for describing operations on that data : – queries and data modifications.

Relational Model

• Relations are tables representing information . – The set of tuples for a given relation is an instance of that

relation

• Columns are headed by attributes ; – each attribute has an associated domain , or data type.

• Rows are called tuples , and – a tuple has one component for each attribute of the

relation.

Schemas

• A relation name , together with the attributes of that relation and their types , form the relation schema.– Movies(title, year, length, genre)

• A collection of relation schemas forms a database schema.

• Particular data for a relation or collection of relations is called an instance of that relation schema or database schema.

Keys

• An important type of constraint on relations is the assertion that an attribute or set of attributes forms a key for the relation. – known as key constraints

• No two tuples of a relation can agree on all attributes of the key , – although they can agree on some of the key attributes.

Semistructured Data Model

• In this model , data is organized in a tree or graph structure .

• XML is an important example of a semistructured data model – Tags similar to those used in HTML define the role played

by different pieces of data• much as the column headers do in the relational model

SQL

• The language SQL is the principal query language for relational database systems .

• The current standard is called SQL-99 .

• Commercial systems generally vary from this standard but adhere to much of it .

Relation implementation

• Stored relations, called tables, exist in the database and can be modified by changing its tuples, as well as queried.

• Views are relations defined by a computation– these relations are not stored, but are constructed, in whole

or in part, when needed

• Temporary tables are constructed by SQL during the execution of queries and data modification– after the transaction these are thrown away

Data Definition

• SQL has statements to declare elements of a database schema .

• The CREATE TABLE statement allows us to declare the schema for stored relations – specifying the attributes , their types , default values , and

keys.

Altering Relation Schemas

• We can change parts of the database schema with an ALTER statement .

• These changes include – adding and removing attributes from relation schemas and – changing the default value associated with an attribute .

• We may also use a DROP statement to completely eliminate relations or other schema elements.

Modifying relations examples

DROP TABLE R;Relation R is no longer part of the database schema, and its tuples can no longer be accessed

ALTER TABLE MovieStar ADD phone CHAR(16);ALTER TABLE MovieStar DROP birthdate;

CREATE TABLE MovieStar ( name CHAR(30), address VARCHAR(255), gender CHAR(1), birthdate DATE, PRIMARY KEY (name));

Algebra

• An algebra consists of operators and atomic operands.

• In relational algebra, the atomic operands are – Variables that stand for relations– Constants which are finite relations

Relational Algebra

• This algebra underlies most query languages for the relational model.

• Its principal operators are – union , intersection, difference , selection , – projection , Cartesian product , – natural join , theta-join , and – renaming .

Section 2.4 Relational algebra and SQL

• From the text– Relational algebra is not used today as a query language in

commercial DBMS’s, although some of the early prototypes did use this algebra directly.

– SQL incorporates relational algebra at its center, and many SQL programs are really “syntactically sugared” expressions of relational algebra.

Some SQL

CREATE TABLE Product (maker CHAR(30),model CHAR(10) PRIMARY KEY,type CHAR(15)

);

CREATE TABLE PC (model CHAR(30),speed DECIMAL(4,2),ram INTEGER,hd INTEGER,price DECIMAL(7,2)

);

CREATE TABLE Laptop (model CHAR(30),speed DECIMAL(4,2),ram INTEGER,hd INTEGER,screen DECIMAL(3,1),price DECIMAL(7,2)

);

CREATE TABLE Printer (model CHAR(30),color BOOLEAN,type CHAR (10),price DECIMAL(7,2)

);

Selection and Projection

• The selection operator produces a result consisting of all tuples of the argument relation that satisfy the selection condition .

• Projection removes undesired columns from the argument relation to produce the result

Some Data for Product

M M Ta o yk d pe e er l

A 1001 pcA 1002 pcA 1003 pcA 2004 laptopA 2005 laptopA 2006 laptopB 1004 pcB 1005 pcB 1006 pcB 2007 laptopC 1007 pcD 1008 pcD 1009 pc

D 1010 pcD 3004 printerD 3005 printerE 1011 pcE 1012 pcE 1013 pcE 2001 laptopE 2002 laptopE 2003 laptopE 3001 printerE 3002 printerE 3003 printerF 2008 laptopF 2009 laptopG 2010 laptopH 3006 printerH 3007 printer

Some data for PC

model speed ram hd price1001 2.66 1024 250 21141002 2.10 512 250 9951003 1.42 512 80 4781004 2.80 1024 250 6491005 3.20 512 250 6301006 3.20 1024 320 10491007 2.20 1024 200 5101008 2.20 2048 250 7701009 2.00 1024 250 6501010 2.80 2048 300 7701011 1.86 2048 160 9591012 2.80 1024 160 6491013 3.06 512 80 529

What PC models have a speed of at least 3.00?

R1 := σspeed ≥ 3.00 (PC)

R2 := πmodel(R1)

model

1005

1006

1013

Some data for Laptop

model speed ram hd screen price2001 2.00 2048 240 20.1 36732002 1.73 1024 80 17.0 9492003 1.80 512 60 15.4 5492004 2.00 512 60 13.3 11502005 2.16 1024 120 17.0 25002006 2.00 2048 80 15.4 17002007 1.83 1024 120 13.3 14292008 1.60 1024 100 15.4 9002009 1.60 512 80 14.1 6802010 2.00 2048 160 15.4 2300

Which manufacturers make laptops with a hard disk of at least 100GB?

R1 := σhd ≥ 100 (Laptop)

R2 := Product (R1)R3 := πmaker (R2)

maker

E

A

B

F

G

Joins

• We join two relations by comparing tuples , one from each relation.

• In a natural join , we splice together those pairs of tuples that agree on all attributes common to the two relations.

• In a theta-join , pairs of tuples are concatenated if they meet a selection condition associated with the theta-join.

Constraints in Relational Algebra

• Many common kinds of constraints can be expressed as – the containment of one relational algebra expression in

another , or – as the equality of a relational algebra expression to the

empty set.