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Schemas, Subschema and Instances

Schema:-

The plan (or formulation ) of the databaseis known as schema.

Schema gives the names of the entitiesand attributes.

Schema speci es the relationship amongthem.

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It is a framework into which the values ofthe data items (or elds) are tted.

The plans or the format of schemaremains the same.

ut the values tted into this formatchanges from instance to instance.

The following e!ample speci es theconte!t of the schema of "#s $ %& amanufacturing compan'.

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Product

Prod-id Prod-desc Unit-cost CustomerCust-id

Cust-name

Cust-street

Cust-city

Cust-bal

Sales

Cust-id Prod-id Prod- city Prod-price

Schema diagram for

sales record

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The schema diagram displa's the structureof each record t'pe but not the actualinstances of records.

ach ob ect in the schema& for e!ample&*+, %T& % ST," +& S$/ S are calledschema construct.

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Schema name is S$/ S-+ %,+t'pe *+, %T0record

*+, -I :String1 *+, - S%:String1 2IT-%,ST:integer1 end

t'pe % ST," +0record % ST-I :integer1 % ST-2$" :String1 % ST-ST+ T0String1 % ST-%IT30String1 % ST- $/0integer1 end

t'pe S$/ S0record

% ST-I 0integer1 *+, -I 0String1 *+, -4T30integer1 *+, -*+I% 0integer1 end

$ttributes (orelds& or dataitems)

$ttributes (orelds& or dataitems)

$ttributes (orelds& or data

items)

Schema de ned using databaselan ua e

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$ database s'stem can have severalschemas partitioned according to the levelsof abstraction.

In general Schema can be categorised intwo parts.

5) /ogical Schema

6) *h'sical Schema

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The logical schema is concerned with

e!ploiting the data structures o7ered b' a"S in order to make the schema

understandable to the computer.

The physical schema deals with themanner in which the conceptual databaseshall get represented in the computer as astored database.

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The logical schema is the most importantas programs use it to constructapplications.

The ph'sical schema is hidden beneath thelogical schema and can usuall' be changedeasil' without a7ecting applicationprograms.

"Ss provide / and S / in order tomake the speci cation of both the ph'sicaland logical schema eas' for the $.

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Subschema:-

It is a subset of the schema and inheritsthe same propert' that a schema has.

The plan(or schema) for a view is oftencalled subschema.

Subschema refers to an applicationprogrammer8s (user8s) view of the data itemt'pes and record t'pes& which he or sheuses.

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Subschema gives the users a window

through which he or she can view onl'that part of the database& which is ofinterest to him.

i7erent application programs can havedi7erent view of data.

Individual application programs canchange their respective subschemawithout e7ecting subschema views ofothers.

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The "S software derives the

subschema data re9uested b' applicationprograms from schema data.

The database administrator ( $)

ensures that the subschema re9uested b'application programs is derivable fromschema.

The change in ph'sical organisation ofdata& application programs for subschemaneed not be changed or modi ed.

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I2ST$2% S:-

;hen the schema framework is lled

in the data item values or the contentsof the database at an' point of time (orcurrent contents)& it is referred to as aninstance of the database.

The term instance is also called asstate of the database or snapshot.

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PRO UC!-I PRO UC!- "SC U#I!-COS!

$56 ST / $/"I+$? =@@@

6@@

==

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CUS!-I PRO -I )!& U#I!-PRIC"

5@@5 $56 5@@ C&F@@

5@@@ 6@ =&@@@

5@5@ == 55@ >&>@@

5@@5 $6DE

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!*R""-("+"( $!$ '$S"

$RC*I!"C!UR" The atabase Task Group ( TG)

appointed b' the %onference on atas'stems and /anguages (%, $S3/)&produced a proposal for general architecturefor database s'stems.

The TG proposed a two-tier architecturewith a s'stem view called the schema andusers views called subschemas.

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In 5DF>& $2SI-S*$+% ($merican 2ationalStandards Institute- Standard *lanning and+e9uirements %ommittee) produced athree-tier architecture with a s'stemcatalog.

The three-tier architecture consists of thefollowing three levels:

5) Internal level6) %onceptual level

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serview 5

serview 5

serview 5

serview 5

*******

%onceptual Schema

Internal Schema

*h'sicalatabase

Ailele

le

le

le

!ternal Schema!ternal

level( e ned b'user orapplication

program)%onceptual level( e ned b' $)

Internal /evel( e ned b'$)

*h'sical/evel

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The view at each of the levels is describedb' a scheme or schema.

The term view& scheme and schema areused interchangeabl'.

The / is used to de ne the %onceptualand e!ternal schemas.

The S4/ are used to describe the aspects

of the ph'sical (or internal schema).

Information about the internal&conceptual and e!ternal schemas is stored

in the s'stem catalog.

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The following e!ample speci es three levelscorresponding to three views (namel'internal& conceptual and e!ternal views).

Customer

%ust-id %ust-name

%ust-street

%ust-cit' %ust-bal

T'pe % ST," +0record% ST-I 0integer1

% ST-2$" 0string1% ST-ST+ T0string1% ST-%IT30string1% ST- $/0integer1

end

Integrated recordde nition of % ST," +record

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The second level of abstraction is theconceptual or global ie .

CUS!O%"R

% ST-I :integer (=)

% ST-2$" :string (6@)

% ST-ST+ T:string (=@)

% ST-%IT3:string (5@)

% ST- $/:integer (E)

$t the conceptual or global level& the database containsinformation concerning an entit' t'pe called % ST," +. achindividual customer has a % ST-I (= digits)& % ST-2$" (6@characters)& % ST-ST+ T(=@ %haracters) and % ST- $/(E digits).

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The third level is the highest level of

abstraction seen b' user or applicationprogram and is called the e!ternal view oruser view.

The conceptual view is the sum total of userview or e!ternal view of data.

CUS!-#$%"

CUS!-CI!& CUS!-I

CUS!-#$%"

CUS!-'$(

S +S I ;5 S +S I ;6

The user view or logical or e!ternal schema of the database inwhich one view contains two elds or data items and other view

contains three elds or data items . There is onl' oneconceptual schema and one internal

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Internal (e el:-

It is the physical representation of thedatabase on the computer and this ieis found at the lo est le el ofabstraction of database.

!he le el indicates ho the data illbe stored in the database and describes

the data structures, le structures andaccess methods to be used by thedatabase.

It describes the ay the '%S and

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Internal level is concerned with the

following activities:

5) Storage space allocation for data andstorage

6) +ecord descriptions for storage withstored siJes for data items.

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The process arriving at a good internal (orph'sical) schema is called physicaldatabase design.

The internal schema is written using S)( orinternal data de nition language (internal /).

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Conceptual (e el:-It is the middle level in the three-tier

architecture.

$t this level of database abstraction& allthe database entities and relationshipsamong them are included.

It contains the logical structure of theentire database as seen b' the $.

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It is complete view of the datare9uirements of the organisation that isindependent of an' storage considerations.

The conceptual schema de nesconceptual view.

It is also called the logical schema.

There is onl' one conceptual schema perdatabase.

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%onceptual level is concerned with thefollowing activities:-

5) $ll entities& their attributes and theirrelationships.

6) %onstraint on the data

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=) %hecks to retain data consistenc'and integrit'.

>) Securit' information.

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"1ternal le el:-

It is the users view of the database.

It is the highest level of data abstraction.

,nl' those portions of the database ofconcern to a user or application programare included.

In e!ternal level& the di7erent views ma'have di7erent representations of the

same data.

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$dvantages of Three-tier $rchitecture:-

The main ob ective of the three-tier

database architecture is to isolate eachuser8s view of the database from the wa'the database is ph'sicall' stored orrepresented.

5) ach user is able access same databut have a di7erent customiJed view of thedata as per their own needs.

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6) The user is not concerned about the ph'sicaldata storage details.

) The $ is able to change the conceptualstructure of the database without a7ecting allusers.

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%haracteristics of Three-tier $rchitecture:-

The following table shows degree ofabstraction& characteristics and t'pe of

"S used for the three levels.

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2eatures

$bstraction(e el

Physicalle el

Internallevel

Conceptual le el

"1ternal (e el

egrees of$bstraction /ow Medium ?igh "edium

egreeof3no

ledgere4

uired'ydatabase

?ardwareandsoftwaredependent

Hardwar e ands/wdepende

nt

?#w ands#wdependent

?#w andS#wdependent

?ierarchical"S

$ttentionre9uiredaboutph'sical-level details

Attentionrequiredabout

physicalleveldetail

$ttentionre9uiredaboutph'sical-lev eldetail

$ttentionre9uiredaboutph'sicalleveldetail

2etwork"S

$ttentionre9uiredabout

ph'sicallevel details

,,

&& &&

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$!$ I# "P"# "#C"

It is a ma or ob ective of implementing"S in an organisation.

It is the characteristics of a databases'stem to change the database schema atone level without having to change theschema at the ne!t higher level.

5) *h'sical ata Independence

6) /ogical ata Independence

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Physical ata Independence:-

Immunit' of the conceptual (ore!ternal) schemas to changes in the internalschema is referred to as ph'sical data

independence.

(ogical ata Independence:-

Immunit' of the e!ternal schemas (orapplication programs) to changes in theconceptual schema is referred to as logicaldata independence.

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"$**I2GS:-

The process of transforming re9uestsand results between the three levels arecalled mappings.

The "S is responsible for thismapping between internal& conceptual& ande!ternal schemas.

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The Three-tier architecture model providesthe following two-stage mappings.

5) %onceptual#Internal mapping

6) !ternal#%onceptual mapping

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The conceptual Schema is related to the internalschema through conceptual#internal mapping.

The conceptual#internal mapping de nes thecorrespondence between the conceptual view

and stored database.

It also allows an' di7erences in entit' names&attribute names& attribute orders& data t'pes and

so on& to be resolved.

$n' change in the structure of the storeddatabase& the conceptual#internal mapping is alsochanged accordingl' b' the structure of the $.

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There could be one mapping betweenconceptual and internal levels

There are several mappings betweene!ternal and conceptual levels.

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$!$ %O "(S:-

$ model is an abstraction process .

$ data model also called database modelis a mechanism that provides this abstractionfor database application.

It represents the organisation itself.

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$ data model is a collection of mathematicall'

well-de ned concepts that help an enterpriseto consider and e!press the static andd'namic properties of data intensiveapplications.

It consists of the following

5) Static properties for e!1 ob ects&attributes& relationships.

6) 'namic properties for e!1 operations or

rules de ning new database states.

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ata "odels can be broadl' classi ed into thefollowing three categories:

5) +ecord based data models.

6) ,b ect-based data models.

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Record 'ased ata %odels:-

The' are used to specif' the overall

logical structures of the database.

In this model the database consists of anumber of !ed-format records possibl' of

di7erent t'pes.

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There are three record based data models

5) ?ierarchical data model

6) 2etwork data model

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Ob5ect 'ased ata %odels:-

The' are used to describe data and itsrelationships.

It uses concepts such as entities& attributesand relationships.

It has Ke!ible data structuring capabilities.

ata integrit' constraints can be e!plicitl'speci ed.

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The following are the common t'pes of,b ect-based data models:-

5) ntit'-+elationship model.

6) Semantic "odel.

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?ierarchical ata "odels:-

The model is represented b' an upside-down tree.

The following gure shows the model

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$

$ $

$ $

/evelL@ : +oot *arent

(node)

/evelL5: +oot %hildren

(Segments)

/evelL6: Segments(level-5 children)