Transcript The Relational Model - University of Houston

Mapping E/R Diagrams to
Relational Database Schemas
Second Half of Chapter 3
Mapping E/R to RM, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
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E/R  Relation Model (Example)
since
name
dname
ssn
Employees
(0,*)
(1,1)
Manages
(0,*)
hours_worked
hourly_wages
did
lot

Hourly_Emps

Works_In
budget
Departments
(0,*)
contractid
Contract_Emps
Mapping E/R to RM, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
since
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Dr. Eick’s Default Mapping
E/R  Relational Data Model
1.
2.
3.
For each entity type create a relation with the attributes associated
with the entity type. Choose a primary key for the defined relation; if
the entity type is weak, delay choosing primary keys until all
identifying relationships are mapped.
For each relationship type create a relation that contains the roles
as well as the attributes of the relationship type. Define referential
integrity constraints with respect to the mapped roles. Exception: If
there is a (1,1) cardinality constraint do not generate a separate
relation, but rather associate the relationship information with the
relation of this participating entity type.
For each sub-type create a relation that contains the attributes of the
entity type as well as the primary key of the most general super class
of this entity type (which also will be the primary key of the
generated relation). Define referential integrity constraints with
respect to the direct super class of the mapped entity type.
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Mapping E/R to RM, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
Logical DB Design: ER to Relational
1. Entity Types to Tables.
ssn
name
Employees
lot
CREATE TABLE Employees
(ssn CHAR(9),
name CHAR(20),
lot INTEGER,
PRIMARY KEY (ssn))
Mapping E/R to RM, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
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2. Relationship Types to Tables
• In translating a relationship
set to a relation, attributes of
the relation must include:
• Keys for each
participating entity set
(as foreign keys).
• This set of attributes
forms a superkey for
the relation.
• All descriptive attributes.
CREATE TABLE Works_In(
ssn CHAR(9),
did INTEGER,
since DATE,
PRIMARY KEY (ssn, did),
FOREIGN KEY (ssn)
REFERENCES Employees,
FOREIGN KEY (did)
REFERENCES Departments)
Mapping E/R to RM, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
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Translating ER Diagrams
with Key Constraints
• Map relationship to a
table:
• Note that did is
the key now!
• Separate tables for
Employees and
Departments.
• Since each
department has a
unique manager, we
could instead
combine Manages
and Departments.
CREATE TABLE Manages(
ssn CHAR(9),
did INTEGER,
since DATE,
PRIMARY KEY (did),
FOREIGN KEY (ssn) REFERENCES Employees,
FOREIGN KEY (did) REFERENCES Departments)
CREATE TABLE Dept_Mgr(
did INTEGER,
dname CHAR(20),
budget REAL,
manager CHAR(9),
since DATE,
PRIMARY KEY (did),
FOREIGN KEY (manager) REFERENCES Employees)
Mapping E/R to RM, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
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Review: Weak Entities
• A weak entity can be identified uniquely only by
considering the primary key of another (owner) entity.
• Owner entity set and weak entity set must participate in a
one-to-many relationship set (1 owner, many weak entities).
• Weak entity set must have total participation in this
identifying relationship set.
name
ssn
lot
Employees
(0,*)
Parent
cost
Policy
Policy
Mapping E/R to RM, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
dname
(1,1)
age
Dependents
Dependents
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Translating Weak Entity Types
• Weak entity set and identifying relationship
set are translated into a single table --- it has a
(1,1) cardinality constraint.
CREATE TABLE Dep_Policy (
dname CHAR(20),
age INTEGER,
cost REAL,
parent CHAR(9) NOT NULL,
PRIMARY KEY (pname, ssn),
FOREIGN KEY (parent) REFERENCES Employees,
ON DELETE CASCADE)
Mapping E/R to RM, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
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name
ssn
Review: ISA Hierarchies
hourly_wages
in C++, or other PLs,
attributes are inherited.
•If we declare A ISA B, every A
entity is also considered to be a B
entity.
lot
Employees
hours_worked
ISA
•As
contractid
Hourly_Emps
Contract_Emps
• Overlap constraints: Can Joe be an Hourly_Emps as well as
a Contract_Emps entity? (Allowed/disallowed)
• Covering constraints: Does every Employees entity also have
to be an Hourly_Emps or a Contract_Emps entity? (Yes/no)
Mapping E/R to RM, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
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3. Translating ISA Hierarchies to Tables
• General approach:
• 3 relations: Employees, Hourly_Emps and Contract_Emps.
• Hourly_Emps: Every employee is recorded in
Employees. For hourly emps, extra info recorded in
Hourly_Emps (hourly_wages, hours_worked, ssn); must
delete Hourly_Emps tuple if referenced Employees
tuple is deleted).
• Queries involving all employees easy, those involving
just Hourly_Emps require a join to get some attributes.
• Alternative: Just Hourly_Emps and Contract_Emps.
• Hourly_Emps: ssn, name, lot, hourly_wages, hours_worked.
• Each employee must be in one of these two subclasses.
Mapping E/R to RM, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
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Dr. Eick’s Default Mapping
E/R  Relational Data Model
1.
2.
3.
For each entity type create a relation with the attributes associated
with the entity type. Choose a primary key for the defined relation; if
the entity type is weak, delay choosing primary keys until all
identifying relationships are mapped.
For each relationship type create a relation that contains the roles
as well as the attributes of the relationship type. Define referential
integrity constraints with respect to the mapped roles. Exception: If
there is a (1,1) cardinality constraint do not generate a separate
relation, but rather associate the relationship information with the
relation of this participating entity type.
For each sub-type create a relation that contains the attributes of the
entity type as well as the primary key of the most general super class
of this entity type (which also will be the primary key of the
generated relation). Define referential integrity constraints with
respect to the direct super class of the mapped entity type.
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Mapping E/R to RM, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
ssn name
occurred
(0,*)
husband
Male

wife
(0,*)
Person
Female
(1,1)
Wedding
(0,*)
(0,*)
Is-insured
Company
location
Con#
amount
to
from
E/R Diagram to be mapped
name
Mapping of the Multi-Wedding E/R
Diagram to a Relational Schema
Person(ssn,name)
Male_Person(ssn)
Company(name,location)
Female_Person(ssn)
Wedding(husband,wife,from,to)
Is-Insured(hssn,wssn,from,company, amount, Con#)
Correct Syntax: FOREIGN KEY (hssn,wssn,from) REFERENCES
Wedding(husband,wife,from)
Mapping E/R to RM, R. Ramakrishnan and J. Gehrke with Dr. Eick’s additions
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