09-ER to Relational Mapping

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ER to Relational

Mapping

2002, National Centre for Software Technology, Bangalore.

ER to Relational 1999-2002 NCST 1


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Translating the E-R Model

E-R model captures the database at the conceptual

level - translating the model into the relational

model (or any other model) yields the actualdatabase.

a Data-Definition Language (DDL) is used to set

up the actual database schema.

ER to Relational 1999-2002 NCST 2


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ER to Relational 1999-2002, NCST 3

trainee

sponsor

is_sponsored_by

Every trainee has one and only onesponsor and every sponsor sponsors

one and only one trainee.

One-to-One

Mandatory

Sp_Id ....... .... ..........

EmpId ....... .... Sp_Id

Not Null!


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Every dept has one and only onemanager; an employee can be the

manager of at most one dept.

One-to-One

Optional

Emp_Id ........ ..... ............

DeptId ....... .... Emp_Id

department

employee

manages

Not Null!


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Some of the consultants areprovided PCs at their desks.

One-to-One

Optional

PC

consultant

provided_with

PC_Id ....... .... ..........

EmpId ....... .... PC_Id

Null Allowed!


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Every employee is alloted adepartment. A department

however, staffs none, one or manyemployees.

One-to-Many

Optional

Dept ....... .... ..........

EmpId ....... .... Dept

department

employee

staffs

Not Null!



A consultant can have at most onesecretary. A secretary may work for

several consultants.

One-to-Many

Optional

Sec_Id ....... .... ..........

C_Id ....... .... Sec_Id

secretary

consultant

works_for

Null Allowed!



A professional association may havenone, one or more consultants as

members. Similarly, a consultant may be

a member of none, one or manyprofessional associations.

Many-to-Many

Optional

PA_Id ....... .... ..........Prof_asso

c

consultant

is_member_of C_Id ....... ....PA_Id

C_Id ....... ....

is_member_of



Recursive

One-to-One

Mandatory

trainee

is_partner_of

EmpId ....... .... Prtnr_Id

Every trainee has another trainee asa partner.

references

Not Null!



An employee may have anotheremployee as a spouse.

Recursive

One-to-One

Optional

employee

is_married_to

EmpId ....... .... Spouse_Id

references

Null

Allowed.



Consultants are divided into groupswith each group having a leader.

Recursive

One-to-Many

Optional

consultant

leads

EmpId ....... .... Ldr_Id

references

Null

Allowed.



Several projects employ similar skillsets.

i.e., projects share skills.

Recursive

Many-to-Many

Optional

project

shares_skills_with

Proj_Id ....... ....

shares_skills_with

Proj1Id ....... ....Proj2Id SkillId



Consultants use casebooks forprojects. Each consultant uses adifferent casebook for different

projects.

Ternary

One-One-One

Mandatory

consultant

casebook project

Proj_Id ....... ....Book_Id ....... ......

EmpId ....... ....

uses_casebook uses_casebookEmpId Proj_Id Book_Id



Employees use a wide rangeof different skills for each

project.

Ternary

Many-Many-Many

Optional

employee

skills project

Proj_Id ....... ....Skill_Id ....... ......

EmpId ....... ....

uses_skillEmpId Proj_Id Skill_Id

uses_skill


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A consultant is either anaccountant or a lawyer but not

none and not both.

Specialisation

Hierarchies

consultant

Emp_Id ....... .....

Emp_Id ....... ......

1

lawyeraccountant

(d)

accountant

lawyer

does not map toa table


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A consultant is either anaccountant or a lawyer but not

none and not both.

Specialisation

Hierarchies2

Emp_Id ....... ...... acct .... ... lwyr .... ... Type

consultant

A/L

consultant

lawyeraccountant

(d)


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A consultant is either anaccountant or a lawyer or

none, but not both.

Specialisation

Hierarchies

Emp_Id ....... .....

Emp_Id ....... ......

3

accountant

lawyer

Emp_Id ....... .....

consultantconsultant

lawyeraccountant

(d)


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A consultant is either anaccountant or a lawyer or

none or both

Specialisation

Hierarchies4

Emp_Id ....... ...... acct .... ... lwyr .... ... A L

consultant

Boolean

consultant

lawyeraccountant

(o)


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a relationship between two entities implies a

migration of key from one entity to another.

ER to Relational : a recap

generally, every entity translates into a distinct

table.

in a 1:many relationship, the key migrates to the

entity at whose end cardinality is >1.



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ER to Relational : a recap

a many:many relationship is best avoided; where it

does exist, the relation itself maps to a distinct table.

table capturing the many:many relationship should

preferably take the same name as the relationship.



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Data Definition Language

SQL : Structured Query Language.

Components of SQL:

Data Definition Language Data Manipulation Language

Transaction Control Component

and others.......

Data Definition Language (DDL) is used to

set up the schema.


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DDL Syntax

SQL: the standard

case insensitive

SQL: implementations

free-format

SQL*Plus, mySQL,

SQL Server . . . .

SQL2

(SQL92)

v/s

Although the implementations are built around the

SQL standard, they have their own nuances.

support for ad-hoc queries


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DDL Syntax

create table tablename(col1_name datatype,

col2_name datatype,

......coln_name datatype)

where datatype is a valid data-type supported by the DBMS product

in use : char, varchar2, date, long, etc.


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DDL : Example

create table employee(emp_id varchar2(10),

emp_fname varchar2(20),

emp_lname varchar2(20),emp_address varchar2(80),

emp_city varchar2(15),

emp_email varchar2(30))


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Enforcing Integrity Constraints

Integrity Constraints capture business semantics;

modeled by the relationships.

Declarative constraints enforce integrity at the

field level or row level or table level

e.g. column sexcan take values Mor Fonly

columns empnoand datepdconstitute the

primary for the table


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Enforcing Integrity Constraints

Triggers*help enforce integrity constraints at

the database/table/row level (although it can also

be used for enforcing integrity at the field level)

e.g. the total outstandings of a customer should notexceed the credit limit specified for him.

* triggers are procedural, and fire (execute) when a specified event

occurs - like an update on a row, or a deletion of row, etc..


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Declarative Constraints

Enforcing entity integrity (primary key)

create table employee(emp_id varchar2(10) primary key,

emp_name varchar2(20),emp_address varchar2(80));

create table salary_pmts (

sal_emp varchar2(20),

sal_datepd date,sal_amount number,

primary key (sal_emp, sal_datepd));


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Enforcing other business rules

create table employee(emp_id varchar2(10) primary key,emp_name varchar2(20),

emp_address varchar2(80)

emp_sex varchar2(1)

check ( sex in (M,F)),

emp_birthdt date,

emp_joindt date,

check ((emp_joindt - emp_birthdt/365)>18));

all examples use SQL*Plus


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References

Fundamentals of Database Systems

R. Elmasri , S. NavatheAddison Wesley

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09-ER to Relational Mapping

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