book chapter 3 (part 2 )

Book Chapter 3 (part 2 )

From ER to Relational Model

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Relational Data Model Relational Query Language Integrity Constraints ER to RelationalLogical DB Design: ER to

Relational

Translate Entity sets to tables:

CREATE TABLE Employees (ssn CHAR(11), name CHAR(20), lot INTEGER, PRIMARY KEY (ssn))

Employees

ssnname

lot

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Relational Data Model Relational Query Language Integrity Constraints ER to RelationalTranslate Relationship Sets

to Tables

lot

dname

budgetdid

sincename

Works_In DepartmentsEmployees

ssn

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to Tables Attributes of relation

include: Keys for each

participating entity set

All descriptive attributes.

CREATE TABLE Works_In( ssn CHAR(11), did INTEGER, since DATE)

lot

dname

budgetdid

sincename


ssn

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to Tables Foreign Keys:

• Keys for each participating entity set (?)

Keys:• This set of

attributes forms superkey for relation (?)

CREATE TABLE Works_In( ssn CHAR(11), did INTEGER, since DATE, PRIMARY KEY (ssn, did), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments)

lot

dname

budgetdid

sincename


ssn

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Relational Data Model Relational Query Language Integrity Constraints ER to Relational

Key Constraints

Translation to relational model?

Many-to-Many1-to-1 1-to Many Many-to-1

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Review: Key Constraint Each dept has at

most one manager, according to key constraint on Manages.

Each department appears only once in relationship


1-to Many

dname

budgetdid

since

lot

name

ssn

ManagesEmployees Departments

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Translate Key Constraint : Approach I.

Separate tables for Employees and Departments.

Note that did is key now!

TABLE Dept(…)TABLE Employee (…)

CREATE TABLE Manages( ssn CHAR(11), did INTEGER, since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments)

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Translate Key Constraint: Approach II.

Combine Manages and Departments into one relation.

Each department has a unique manager

TABLE Employee (…)

CREATE TABLE Dept_Mgr( did INTEGER, dname CHAR(20), budget REAL, ssn CHAR(11), since DATE,………………. )

dname

budgetdid

since

lot

name

ssn


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Translate Key Constraint: Approach II.

Combine Manages and Departments into one relation.

Each department has a unique manager, if any.


CREATE TABLE Dept_Mgr( did INTEGER, dname CHAR(20), budget REAL, ssn CHAR(11), since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees)

dname

budgetdid

since

lot

name

ssn


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Other Cases of Key Constraints


Many-to-Many1-to-1 1-to Many Many-to-1

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Relational Data Model Relational Query Language Integrity Constraints ER to RelationalReview: Participation

Constraints

The “must have” constraint (not-null value)?

lot

name dnamebudgetdid

sincename dname

budgetdid

since

Manages

since

DepartmentsEmployees

ssn

Works_In

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Relational Data Model Relational Query Language Integrity Constraints ER to RelationalParticipation + Key

Constraint. Every department must have a manager !

• Every did value in Departments table must appear in a row of the Manages table (with a non-null ssn value!)

lot

name dnamebudgetdid

sincename dname

budgetdid

since

Manages DepartmentsEmployees

ssn

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Relational Data Model Relational Query Language Integrity Constraints ER to RelationalParticipation Constraints in

SQL Approach I.

• every did value in Department appears in a tuple of Managers• corresponding tuple must have a non-null ssn values

• Does this capture with “not-null” work, or not?

TABLE Dept(…)TABLE Employee (…)

CREATE TABLE Manages( ssn CHAR(11) NOT NULL, did INTEGER, since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments,

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SQL Approach I.

• every did value in Department appears in a tuple of Works_In• the corresponding tuple must have a non-null ssn values

TABLE Dept_mgr(…)TABLE Employee (…)

CREATE TABLE Manages( ssn CHAR(11) NOT NULL, did INTEGER, since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments, CHECK ( (SELECT Count(*) FROM Manages) = (SELECT Count(*) FROM Dept) )

Must utilize check constraints !

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Participation Constraints in SQL Approach II.

- capture participation constraints involving one entity set in a binary relationship using combined table.


CREATE TABLE Dept_Mgr( did INTEGER, dname CHAR(20), budget REAL, ssn CHAR(11) NOT NULL, since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees, SEMANTICS ??? )

Does this “non-null” approach now work ?

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Participation Constraints in SQL Approach II.

- capture participation constraints involving one entity set in a binary relationship using combined table.


CREATE TABLE Dept_Mgr( did INTEGER, dname CHAR(20), budget REAL, ssn CHAR(11) NOT NULL, since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees, SEMANTICS ??? )

What should happen if manager-employee is deleted?

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SQL Approach II.

- use the “on action” propagation constraint !

CREATE TABLE Dept_Mgr( did INTEGER, dname CHAR(20), budget REAL, ssn CHAR(11) NOT NULL, since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees, ON DELETE NO ACTION)

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Relational Data Model Relational Query Language Integrity Constraints ER to RelationalMore on Participation

Constraints What about other cases of “must have”

constraint ?

lot

name dnamebudgetdid

sincename dname

budgetdid

since

Manages

since

DepartmentsEmployees

ssn

Works_In

What if we want to capture participation for many-to-many relationships?

Anwer: We need to use CHECK constraints.

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SQL

What if we want to capture participation for three-way relationships?

Anwer: We need to use CHECK constraints.

Observation : Little else can be enforced without resorting to the use of check constraints !

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Review: Weak Entities

A weak entity can be identified uniquely only by considering 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.

lot

name

agepname

DependentsEmployees

ssn

Policy

cost

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Translating Weak Entity Sets Weak entity set and identifying

relationship set are translated into a single table.

CREATE TABLE Dep_Policy ( pname CHAR(20), age INTEGER, cost REAL, ssn CHAR(11) NOT NULL, --- (?), PRIMARY KEY (pname, ssn), --- (?) FOREIGN KEY (ssn) REFERENCES Employees, WHAT SEMANTICS HERE ??? )

What when the owner entity is deleted? Then all owned weak entities must also be deleted !

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Translating Weak Entity Sets

When the owner entity is deleted, all owned weak entities must also be deleted.

CREATE TABLE Dep_Policy ( pname CHAR(20), age INTEGER, cost REAL, ssn CHAR(11), PRIMARY KEY (pname, ssn), FOREIGN KEY (ssn) REFERENCES Employees, ON DELETE CASCADE)

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Review: ISA Hierarchies

Contract_Emps

namessn

Employees

lot

hourly_wages

ISA

Hourly_Emps

contractid

hours_worked

Attributes are inherited

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Translating ISA Hierarchies

What are possible alternatives of mapping IS-A to the relational model ?

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General approach: 3 relations: Employees, Hourly_Emps and

Contract_Emps.Employees ( ssn, name, lot) Hourly_Emps (ssn, hourly_wages, hours_worked);Contract_Emps (ssn, contractid);

Keys?

Foreign keys?

Delete semantics?

- Delete Hourly_Emps tuple if referenced Employees tuple is deleted

- how?

- Put CASCADE semantics on foreign key.

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Alternative: Two tables Namely: Hourly_Emps and Contract_Emps

Hourly_Emps (ssn, name, lot, hourly_wages, hours_worked)Contract_Emps (ssn, name, lot, contractid)

Keys?

Foreign keys?

Delete semantics?

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Pros/Cons : Translating ISA Hierarchies Alternative : Three Relations

Employees ( ssn, name, lot) Hourly_Emps (ssn, hourly_wages, hours_worked);Contract_Emps (ssn, contractid);

Alternative: Two tablesHourly_Emps (ssn, name, lot, hourly_wages, hours_worked)Contract_Emps (ssn, name, lot, contractid)

Pros/cons for three relations:+ Queries involving all employees easy- Queries involving just Hourly_Emps may require a Join.

Pros/Cons for two relations:- Each employee must be in one of these two subclasses. - Avoid joins for subtable queries

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ISA Hierarchy Translation?

Contract_Emps

namessn

Employees

lot

hourly_wages

ISA

Hourly_Emps

contractid

hours_worked

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)

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Another Example

Requirements:

1. A policy cannot be owned by two or more employees.(Key Constraints)

2. Every policy must be owned by some employee (Total participation)

3. Dependents is a weak entity (Weak Entity)

Beneficiary

agepname

Dependents

policyid cost

Policies

Purchaser

name

Employees

ssn lot

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Example of Mapping to Tables Key constraints

allow us to combine (Purchaser with Policies) and (Beneficiary with Dependents.)

Participation constraints lead to NOT NULL constraints.

Policies is a weak entity set: ON DELETE CASCADE

CREATE TABLE Policies ( policyid INTEGER, cost REAL, ssn CHAR(11) NOT NULL, PRIMARY KEY (policyid). FOREIGN KEY (ssn) REFERENCES Employees)

CREATE TABLE Dependents ( pname CHAR(20), age INTEGER, policyid INTEGER, PRIMARY KEY (pname, policyid). FOREIGN KEY (policyid) REFERENCES Policies, ON DELETE CASCADE)

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Summary

ER Modeling : graphical design view Relational Model: A tabular

representation of data. Rules to translate ER to relational model

exist

Later : Yet more design optimizations can and should be applied, after initial relational design has been derived.

book chapter 3 (part 2 )

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