Lecture 10 DBMS

8/12/2019 Lecture 10 DBMS

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Database Systems


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The Relational Model of Data

The term relation is basically just a mathematical

term for a table.

DBMS products that are based on the relationalmodel of datahave come to dominate the

database marketplace.

The introduction of the relational model by E. F.

Codd in 1969-70 was the most important event

in the entire history of the database field.


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The Relational Model -

cont inued C. J. Date was the first to recognize the significance

of the relational model and has been the mostrecognized author, lecturer, researcher, and

consultant of the relational technology He is best known for his book An Introduction to

Database Systems (seventh edition published in2000), which has sold nearly 650,000 copies and is

used by several hundred colleges and universitiesworldwide.


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Terminology

S#:S# SNAME:NAME STATUS:STATUS CITY:CITY

S1

S2

S3

S4

S5

Smith

Jones

Blake

Clark

Adams

20

10

30

20

30

London

Paris

Paris

London

Athens

Relatio

n

Tuple

Attributes

Primary

Key

London,

Tucson,

Paris, ...

S# NAME STATUS CITY

D

omains


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Relational Data Objects:

Domains and Relations

A relationis table

A tuple corresponds to a row of such a tableand attribute to a column

The number of tuples is called the

cardinality and number of attributes arecalleddegree


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Domain

A domain is a pool of values, from which specific

attributes of specific relations draw their actual

values.

Synonym of data type(Typefor short). Can be system-defined, e.g., text, number, currency,

yes/no, date/time, hyperlink, autonumber.

Or user-defined, e.g., POINT.

For example, the domain labeled S# is the set of alllegal supplier numbers, and the set of S# values

appearing in relation S at any given time is

constrained to be some subset of that set.


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Terminologies Used:

Formal Informal

Relation table

Tuple row or record

Cardinality number of rows

Attribute column or field

Degree number of columns

Domain pool of legal values


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Tables

Relational database is a collection of tables

Heading: table name and column names

Body: rows, occurrences of data

StdSSN StdLastName StdMajor StdClass StdGPA

123-45-6789 WELLS IS FR 3.00

124-56-7890 NORBERT FIN JR 2.70

234-56-7890 KENDALL ACCT JR 3.50

Student


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Integrity (Fig. from C.J. Date)

S# SNAME STATUS CITY

S1

S2

S3

S4

S5

Smith

Jones

Blake

Clark

Adams

20

10

30

20

30

London

Paris

Paris

London

Athens

P# PNAME COLOR WEIGHT

P1P2

P3

P4

P5

P6

NutBolt

Screw

Screw

Cam

Cog

RedGreen

Blue

Red

Blue

Red

1217

17

14

12

19

CITY

LondonParis

Rome

London

Paris

London

S#

S1

S1

S1

S1

S1

S1

S2

S2

S3S4

S4

S4

P#

P1

P2

P3

P4

P5

P6

P1

P2

P2P2

P4

P5

QTY

300

200

400

200

100

100

300

400

200200

300

400

S

P

SP


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Integrity

The termintegrity refers to the accuracyand

correctness of data in the database.

The DBMS should automatically enforce

specified integrity constraints.

Entity integrity: primary keys

Each table has column(s) with unique values

Ensures entities are traceable Referential integrity: foreign keys

Values of a column in one table match values from a

source table

Ensures valid references among tables


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Integrity (Cont)

The term integrityrefers to the accuracyorcorrectnessof data in the database.

Integrity constraints guard against accidentaldamage to the database, by ensuring thatauthorized changes to the database do not resultin a loss of data consistency.

A given database might be subject to any number

of integrity constraints of arbitrary complexity. The DBMS needs to be informed of such

constraints, needs to enforce them somehow.

Basically by rejecting any update that would

otherwise violate them.


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Integrity (Cont)

When a new constrain is declared,

The system must first make sure the database currently

satisfies it;

If not, the new constraint is rejected; Otherwise it is accepted and enforced from that point

forward.

We can get rid of existing constraints.


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Types of Constraints

Type (domain) Constraint:

specifies the legal values for a given type.

Attribute constraint:

specifies the legal values for a given attribute.

Table (relation) constraint:

specifies the legal values for a given table.

Multiple attributes may be involved.

Database constraint:

specifies the legal values for a given database.

Multiple tables may be involved.


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Type or Domain Constraints

Type constraints are the most elementary

form of integrity constraint.

A type constraints is an enumeration of the

legal value of the type, a specification of the

values that make up the type in question.

Type constraints are checked immediately.


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Type or Domain Constraints

(Cont.)

The checkclause in SQL permits domains to be

restricted:

Use checkclause to ensure that an hourly-wage

domain allows only values greater than a specifiedvalue.

create domainhourly-wage numeric(5,2)

constraintvalue-test check(value > = 4.00)

The domain hourly-wageis declared to be a decimalnumber with 5 digits, 2 of which are after the decimal

point

The domain has a constraint that ensures that the

hourly-wage is greater than 4.00


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Attribute Constraints

An attribute constraint is just a declaration to theeffect that a specified attribute is of a specified type.

For example:

create tableaccount(branch-name char(15),account-number char(10) not null,balance integer,)

Attribute constraints are part of the definition of theattribute.

Any attempt to introduce an attribute value into thedatabase that is not a type of the relevant type willsimply rejected.

Such a situation should never arise.


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Table Constraint

A table constraint is a constraint on an individual

table.

Example:

Suppliers in London must have status 20.

Two attributes, CITY and STATUS, of table S areinvolved.


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Database Constraint

A database constraint is a constraint that

interrelates two or more distinct tables.

Example:

No suppliers with status less than 20 can supply any

part in a quantity greater than 500.

Two tables, S and SP, are involved.


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Reference:

Chapter No 4 of the book by (C. J. Date)

Home work:

Read the chapter 4 of the book


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Keys

The concept of Keysis very important to the

relational model.

We'll discuss the following types of keys.

Candidate Keys

Primary Keys and Alternative Keys Foreign Keys


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Candidate Keys

Let R be a table. By definition, at any given time,no two tuples in the value of R are duplicates ofone another.

Let K be a set of attributes of R. Then K is acandidate key for R if and only if it satisfies:

(1) Uniqueness: No legal value of R ever contains twodistinct tuples with the same value for K.

(2) Irreducibility (or minimality):

No proper subset of K has the uniqueness property.


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Primary and Alternative Keys

Exactly one candidate key must be chosen as theprimarykey, and the others arealternative keys.

Primary key and alternative keys are both candidatekeys.

For S {S#, SNAME, STATUS, CITY},

if we choose KEY {S#} as the primary key,

then KEY {SNAME} is an alternative key.

For SP {S#, P#, QTY},

We must choose KEY {S#, P#} as the primary key, andthere will be no alternative key.


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Foreign Keys

A foreign keyis a set of attributes of one table

R2 whose values are required to match values

of some candidate keyof some table R1.

{S#} is a foreign keyof SP that referencesthe

primary key{S#} of S.

Any S# value of SP must exist in S.

Similarly, {P#} is another foreign keyof SP that

referencestheprimary key{P#} of P.


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Integrity Rules

Entity integrity No two rows with the same primary key value

No null values in a primary key

Referential integrity Foreign keys must match candidate key of source table

Foreign keys in some cases can be null

The database must not contain any unmatched foreign keyvalues.

If B references A, A must exist.


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Referenced Rows

Referenced row Foreign keys reference rows in the associated primary key

table

Each supply row references supplier and part rows.

Actions on referenced rows Delete a referenced row

Change the primary key of a referenced row

Referential integrity should not be violated


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Possible Actions

Restrict: do not permit action on the

referenced row

Cascade: perform action on related rows

Nullify: only valid if foreign keys accept null

values

Default: set foreign keys to a default value


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Referential Actions on Foreign

Keys

When we delete a tuple from a table, say S, that

is referencedby another table, say SP,

if theprimary key{S#} value of S exists in SP,

there are several choices of referential actions:

ON DELETE CASCADE

The corresponding tuples in SP will be deleted too. ON DELETE RESTRICT

The deletion of the tuple from S is rejected.


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Referential Actions (Cont.)

When we update a tuple from a table, say S, thatis referencedby another table, say SP,

There are similar choices of referential actions: ON UPDATE CASCADE

ON UPDATE RESTRICT

There could be other choices besides thesethree. , e.g., ON UPDATE SET DEFAULT.

ON UPDATE SET NULL.


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Integrity (Fig. from C.J. Date)

S# SNAME STATUS CITY

S1

S2

S3

S4

S5

Smith

Jones

Blake

Clark

Adams

20

10

30

20

30

London

Paris

Paris

London

Athens

P# PNAME COLOR WEIGHT

P1

P2P3

P4

P5

P6

Nut

BoltScrew

Screw

Cam

Cog

Red

GreenBlue

Red

Blue

Red

12

1717

14

12

19

CITY

London

ParisRome

London

Paris

London

S#

S1

S1

S1

S1

S1

S1

S2

S2

S3

S4S4

S4

P#

P1

P2

P3

P4

P5

P6

P1

P2

P2

P2P4

P5

QTY

300

200

400

200

100

100

300

400

200

200300

400

S

P

SP


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SQL Syntax for Actions (Just a

flavor)

CREATE TABLE Enrollment

( OfferNo INTEGER NOT NULL,

StdSSN CHAR(11) NOT NULL,

EnrGrade DECIMAL(3,2),

CONSTRAINT PKEnrollment PRIMARY KEY(OfferNo,

StdSSN),

CONSTRAINT FKOfferNo FOREIGN KEY (OfferNo)REFERENCES Offering

ON DELETE RESTRICT

ON UPDATE CASCADE,

CONSTRAINT FKStdSSN FOREIGN KEY (StdSSN) REFERENCES

Student

ON DELETE RESTRICT

ON UPDATE CASCADE )

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Lecture 10 DBMS

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