lec3a_DataModels

8/9/2019 lec3a_DataModels

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Data Models

How to structure data


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What is a Data Model?

Having formed a model of the enterprise, we now need torepresent the data.

The data model tells us the structure of the database.

Historicall, three data models!

Hierarchical data model

"etwor# data model

$elational data model


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Hierarchical and "etwor# Data Models

Hierarchical and networ# data models have beensuperseded b the relational data model.

$easons!

&ac# of e'pressive power

(.g. one cannot e'press man)to)manrelationships in the hierarchical model

More closel tied to the underling implementation.

Hence, less data independence.

$elational data model has a clean mathematical basis.


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The $elational Model

Due to +odd. (verthing is represented as a relation in the

mathematical sense. lso called tables.

database therefore is a collection of tables, each of

which has a uni-ue name, and each of which is describedb a schema.

n addition, +odd defined a data manipulation language.


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('ample of 0chemas in the $elational Model

('ample of a representation of entit sets!

Student(sid,name,addr)

Course(cid,title,eid)

Empl(eid, ename, deptid)

Dept(deptID, dname, loc)

rimar #es are underlined.

$ecall that a primar #e is one that uni-uel identifies an

entit.

n entit is a row in a table.


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More ('ample 0chemas

$elationship sets between entit sets are alsorepresented in tables.

('ample of a table corresponding to a relationship!

Enrol(sid, cid, grade)

gain, a relationship is represented b a row 3or a tuple4in a table.


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$elational Databases!

6asic +oncepts

Attribute:

column in a table

Domain

The set of values from which the values of an attribute

are drawn. Null value

special value, meaning 7not #nown8 or 7not

applicable8.

Relation schema set of attribute names


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$elational Databases! 6asic +oncepts

Tuple

set of values, one for each attribute in the relation

schema over which the tuple is defined, i.e. a mapping

from attributes to the appropriate domains

Relation instance set of tuples over the schema of the relation


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$elational Databases!

6asic +oncepts

Relational Database

set of relations, each with a uni-ue name

Normalized Relation

relation in which ever value is atomic 3non)

decomposable4. Hence, ever attribute in ever tuplehas a single value.


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Foreign Key

n attribute 3or set of attributes4 in table $1 which also

occurs as the primar #e of relation $2.

$2 is called the referenced relation.

=oreign #es are also called connection #es orreference attributes.


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ntegrit $ules! (ntit +onstraint

ntity constraint ll attributes in a primar #e must be non)null.

Motivation! f the primar #e uni-uel identifies an

entit in an entit set, then we must ensure that we

have all the relevant information


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ntegrit $ules!

$eferential ntegrit

$eferential integrit database cannot contain a tuple with a value for a

foreign #e that does not match a primar #e value in

the referenced relation.

>r, a foreign #e must refer to a tuple that e'ists. Motivation! f referential integrit were violated, we

could have relationships between entities that we do

not have an information about.


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+onverting ($ Diagrams to $elational 0chemas

3tables4

database which conforms to an ()$ diagram can berepresented b a collection of relations 3tables4.

=or each entit set and relationship set there is a uni-ue

relation that is assigned the name of the corresponding

entit set or relationship set. (ach schema has a number of columns 3generall

corresponding to attributes4, which have uni-ue names.

1*


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$epresenting (ntit 0ets as $elations 3tables4

strong entit set reduces to a relation with the attributesof the entit.

+ourse3cid, ctitle4

wea# entit set becomes a table that includes the

primar #e of the strong entit set. ('ample!

0ection3cid, secno, -uota4

1/


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$epresenting $elationship 0ets as

$elations 3tables4

man)to)man relationship set is represented as arelation with attributes for the primar #es of the two

participating entit sets, and an attributes of the

relationship set.

('ample! (nrolls3sid, cid, grade4

1


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Total articipation

The participation of an entit set in a relationship set istotal if each entit in the set occurs in at least one

relationship in that relationship set

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$elations 3tables4

Man)to)one and one)to)man relationship sets thatare total on the man)side can be represented b

adding an e'tra attribute to the 7man8 side,

containing the primar #e of the 7one8 side.

('ample! +ourse3cid,ctitle,lid4

f participation is not total on the 7man8 side,

replacing the relationship table b an e'tra attribute

in the relation corresponding to the 7man8 side couldresult in null values.

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$elations 3tables4

=or one)to)one relationship sets an e'tra attribute can beadded to either of the tables corresponding to the two

entit sets.

The relation corresponding to a relationship set lin#ing a

wea# entit set to its identifing strong entit set isredundant.

('ample! The section entit alread contains the

attributes that would appear in the relation 3i.e. cid and

secno4.

1:


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$epresenting 0pecialiation

Method 1! +reate a relation for the higher)level entit.

+reate a relation for each lower)level entit set,include primar #e of higher)level entit set and localattributes.

('ample! 0tudent3sid,sname4 @raduate3sid,supervisor4 Andergraduate3sid,credits4

Drawbac#! getting information about a student mare-uire accessing two relations 3i.e. both the low)leveland the high)level relation4.

2;


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$epresenting 0pecialiation

Method 2! =orm a relation for each entit set with all local and

inherited attributes. ('ample!

0tudent3sid,sname4

Andergraduate3sid,sname,credits4 @raduate3sid,sname,supervisor4 f specialiation is total, the schema for the generalied

entit set 3student4 is not re-uired to store information. +an be defined as a 7view8 relation containing union of

specialiation relations Drawbac#! some amount of redundanc ma be

introduced in the design.

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$epresenting ggregation

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Customer LoanBorrows

Employee

Loan-Manager

B Loan-Manager (employee_id, customer_id, loan_id)B Relation Borrows is redundant

What is the primary ey in this relation!

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