Course02 - Relational Model

8/12/2019 Course02 - Relational Model

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Database Management Systems 1

COURSE 2

The Relational Model


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Database Management Systems 1 2

Data Models

Hierarchical Model(1965)

Network Model(1965)

Relational Model (1NF) (1970s)

Nested Relational Model (1970s)

Complex Object (1980s)

Object Model (1980)

Object Relational Model (1990s)

New Trend: XML (DTD), XML Schema (1990s)


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Hierarchical Modela collection of recordswhich are connected to one anotherthrough links.

each record hasone parentandmany children

customers

accounts


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Network Model

each record may have many parentsandmany children

database = set of pairs of sets of records


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Relational model - Ideas

Use a simple data structure: the Tablesimple to understand

useful data structure (capture many situations)

leads to useful not too complex query lang.Use mathematics to describe and representrecords and collections of records: the Relation

can be understood formallyleads to formal query languages

properties can be explained and proven


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Relation

A domain is a set of scalar values (i.e. limited toatomic types - integer, string(24), boolean, date,blobs)

A relationor relation schemaRis a list ofattribute names (fields)[A1, A2, , An].

Ralso called the name of the relation

Corresponding to each attribute name Aiis adomain Dicalled the domain of Ai, we write Di=Dom(Ai).


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Relation (cont)

The number of attributes in a relation scheme iscalled the degree or arity of the relation

A relation instance [R] is a subset of the

Cartesian product of the domains of the attributesin the schema

An element of the relation instance, a record, iscalled a tuple. All tuples of a relation are distinct

The number of rows of a relation is thecardinalityof the relation


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Relation ExampleStudents(sid:integer; name:string;email:string; age:integer; gr:integer)

Cardinality = 4, degree = 5, all rows distinct

field name field type

(domain)

sid name email age gr

2833 Jones [email protected] 19 231

2877 Smith [email protected] 20 232

2976 Jones [email protected] 21 233

2765 Mary [email protected] 22 233

relation

schema

relation

instance

relation

instance

relation

tuple
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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Warnings

Very often we will confuse

the relation, its schema, and its instance

the instance and the tablethe attribute, the field and the column

the tuple and the row

Ask for precision if there is ambiguity!


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Relational Database

A database is a set of relations

A database schema is the set of schemas ofthe relations in the database

A database instance is the set of instancesof the relations in the database


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Graphical Representation of Relations

Students(sid:string, name:string, email:string, age:integer,gr:integer)Courses(cid: string, cname: string, credits:integer)

Enrolled(sid:string, cid:string,grade:double)

Teachers(tid:integer; name: string; sal : integer)

Teaches(tid:integer; cid:string)


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Integrity Constraints (ICs)

IC: condition that must be true for anyinstance ofthe database; e.g., domain constraints.

ICs are specified when schema is defined.

ICs are checked when relations are modified.A legalinstance of a relation is one that satisfies allspecified ICs.

DBMS should not allow illegal instances.

If the DBMS checks ICs, stored data is morefaithful to real-world meaning.

Avoids data entry errors, too!


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Primary Key Constraints

A set of fields is a keyfor a relation if :

1. No two distinct tuples can have same values in all keyfields, and

2. This is not true for any subset of the key.

Part 2 false? A superkey.

If theres >1 key for a relation, one of the keys is

chosen (by DBA) to be theprimary key.Possibly many candidate keys, one of which ischosen as theprimary key.


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Foreign Keys, Referential Integrity

Foreign key: Set of fields in one relation that isused to `refer to a tuple in another relation. (Mustcorrespond to primary key of the second relation.)Like a `logical pointer.

E.g. sidis a foreign key referring to Students:Enrolled (sid: string, cid: string,grade: double)

If all foreign key constraints are enforced,

referential integrityis achieved, i.e., no danglingreferences.

e.g. of a data model w/o referential integrity:Links in HTML!


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Graphical Representation of ICs


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Where do ICs Come From?

ICs are based upon the semantics of the real-world enterprise that is being described in thedatabase relations.

We can check a database instance to see if an IC is

violated, but we can NEVERinfer that an IC is trueby looking at an instance.

An IC is a statement about all possible instances!

From example, we know nameis not a key, but theassertion that sidis a key is given to us.

Key and foreign key ICs are the most common;more general ICs supported too.


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Relational Query Languages

A relational database query is a question about thedata, and the answer consist of a new relation.

A major strength of the relational model: supports

simple, powerful queryingof data.Queries can be written intuitively, and the DBMSis responsible for efficient evaluation.

The key: precise semantics for relational queries.

Allows the optimizer to extensively re-order operations,and still ensure that the answer does not change.


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Structured Query Language (SQL)

Developed by IBM (system R) in the 1970sNeed for a standard since it is used by manyvendors

Standards (ANSI):SQL-86

SQL-89 (minor revision)

SQL-92 (major revision) - 1,120 pages

SQL-99 (major extensions) - 2,084 pagesSQL-2003 (SQL/XML new section) - 3,606 pages

SQL-2007 (expansion completed in 2006)


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SQL Levels

Data-manipulation language (DML)Allows users to pose queries

Insert /delete / modify (update) tuples.

Data-definition language (DDL):

Create / destroy / alter relationsand views.

Define integrity constraints(ICs).

Access Control:

Can grant / revoke the right to access and manipulatetables (relations / views).


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DDL Commands Creating Relations

Creates the Studentsrelation. Observe that thetype (domain) of eachfield is specified, and

enforced by the DBMSwhenever tuples areadded or modified.

As another example, theEnrolledtable holdsinformation about coursesthat students take.

CREATE TABLEStudents(sid CHAR(20),

name CHAR(50),

email CHAR(30),

age INTEGER,

gr INTEGER)

CREATE TABLEEnrolled

(sid CHAR(20),

cid CHAR(5),

grade REAL)


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DDLDestroying/Altering Relations

DROP TABLEStudents

Destroys the relation Students. The schemainformation andthe tuples are deleted.

ALTER TABLE Students

ADD COLUMNfirstYear INTEGER

The schema of Students is altered by adding anew field; every tuple in the current instance isextended with a nullvalue in the new field.


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DDL Primary/Candidate KeysPossibly many candidate keys (specified usingUNIQUE), one of which is chosen as theprimary key.

!Used carelessly, an IC can prevent the storage of

database instances that arise in practice!

For a given student andcourse, there is a single

grade.


(sid CHAR(20),

cid CHAR(20),

grade CHAR(2),PRIMARY KEY(sid,cid))


(sid CHAR(20),

cid CHAR(20),

grade CHAR(2),PRIMARY KEY(sid),

UNIQUE (cid, grade))

Students can take only one course,and receive a single grade for thatcourse; further, no two students in

a course receive the same grade.


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DDL Commands Foreign Keys

Only students listed in the Students relation should beallowed to enroll for courses.

CREATE TABLE Enrolled

(sid CHAR(20), cid CHAR(20), grade CHAR(2),

PRIMARY KEY (sid,cid),FOREIGN KEY(sid) REFERENCESStudents )

sid cid grade

1234 Alg1 91235 Alg1 10

1234 DB1 10

1234 DB2 9

sid name email age gr1234 John [email protected] 21 331

1235 Smith [email protected] 22 331

1236 Anne [email protected] 21 332

StudentsEnrolled


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DDL Commands Referential Integrity

SQL-99 and SQL-2003support all 4 options ondeletes and updates.

Default is NO ACTION

(delete/update is rejected) CASCADE(also delete

all tuples that refer todeleted tuple)

SET NULL/SETDEFAULT (sets foreignkey value of referencingtuple)


(sid CHAR(20),

cid CHAR(20),

grade CHAR(2),PRIMARY KEY (sid,cid),

FOREIGN KEY(sid)

REFERENCESStudents

ON DELETE CASCADE

ON UPDATE SET DEFAULT)


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General Constraints

Useful whenmore general ICsthan keys are

involved.Can use queriesto express

constraint.Constraints canbe named.

CREATE TABLE Students

(sid CHAR(20),

name CHAR(50),

email CHAR(30),

age INTEGER,

gr INTEGER,

PRIMARY KEY (sid),

CONSTRAINTageInterv

CHECK (age >= 18ANDage

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Course02 - Relational Model

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