CS3431-B11 1

The Relational Model

Instructor: Mohamed Eltabakh meltabakh@cs.wpi.edu

Announcements Project-Phase 1 is already out on Oct. 28 and due Nov. 4

Homework 1 is out today Nov. 1 and due Nov. 8

TAs Assigned to Project Teams All phases of a given team will be graded by the same TA The list is on the course website under “Homeworks & Projects” and under blackboard

system under “Project Phases” as well

Homeworks & Project Due Date Reducing the number of HWs and project phases by one so the last due data is Dec.

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cs3431 2

One Last Example of ERD !!!

cs3431 3

What is Bad???

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carcar

ColorNameColorName

colorIDcolorID

VINVINMakeMake ModelModel

CustomerCustomer

IDIDDoBDoB NameName

Car-featureCar-feature

FeatureNameFeatureName

containscontains

buysbuys

DateDate

CarMilesCarMiles

AgeAge

PricePrice

LoanLoan

amountamount

numbernumber

BankBank

takestakes

DateDate

= A customer can buy many cars= A customer may take a loan to buy a specific car

From the Previous Example ColorId & ColorName (cause redundancy & inconsistency)

Car can have one feature (wrong cardinality)

Car-feature has one attribute (should not be an entity)

CarMiles should be attached to the car (not to the relationship)

Age should be a derived attribute

A car can be bought by one (or zero) customers (wrong arrow head)

Loan and Car are not linked together (buys should be 3-way) Or create a new entity set “Contract” and link it to the three entity sets

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Relational Model

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Relational Model Another model for describing your data and application

requirements

Currently the most widely used model Vendors: Oracle, Microsoft, IBM

Recent competitions Object Oriented Model: ObjectStore, Oracle XML Databases : native and extensions

What about ER model ER model is the first step in the design ER model is then translated (mapped) to relational model

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Relational Model Structure:

Relations (also called Tables) Attributes (also called Columns or Fields)

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Attributes in Relational Model Attribute Name

Each attribute of a relation has a unique name within the relation

Attribute Domain The set of allowed values for each attribute is called the domain of the attribute E.g., integer, real, date, string {“Male”, “Female”}, etc.

Atomic Attributes Attribute values are required to be atomic

(primitive or derived) Note: multivalued attributes are not atomic Note: composite attributes are not atomic We will see how to handle non-atomic attributes

Special Null Value The special value null is a member of every domain The null value means the values does not exist (unknown)

Relational Schema Schema is the structure of the relation Attributes A1, A2, ..., An of relation R

R=(A1,A2,...,An ) is the relations schema of R

Relational Schema of “customer” is customer = (customer_name, customer_street, customer_city) customer (customer_name, customer_street, customer_city)

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Relational Instance Instance is the current schema plus the current tuples in the relation

The current instance of “customer” has four tuples (rows or records)

Instances are frequently changing E.g., inserting new tuples, deleting or updating existing tuples

Schemas may also change but not frequently

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Arity & Cardinality of Relations

Arity (Degree) of a relation Number of attributes of that relation

Cardinality of a relation Number of tuples in that relation

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Arity = 3Cardinality = 4

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Relations are Unordered

Order of tuples is not important

Order of columns is not important

Think of a relation as a set (or bag) of tuples not as a list of tuples Most current DBMSs allow bags

sNumber sName

1 Dave

2 Greg

Student

sNumber sName

2 Greg

1 Dave

Student

These two relations are equivalent

Keys of Relations Super Keys

Subset of attributes that uniquely identify each tuple

Candidate Keys Minimal super key

Primary Keys Choose any of the candidate keys Represented by “underline” under the selected attributes

Set vs. Bag If a relation has a primary key, then it is a set not a bag

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customer (customer_name, customer_street, customer_city)

Assuming the customer name is unique

How do we create these relations?

SQL Language

How do we create these relations?

SQL Language

Relational Query Languages (SQL Standard) Developed by IBM (system R) in 1970s Standard as used by many vendors (portable)

SQL (Structured Query Language)

Standards: SQL-86 SQL-89 (minor revision) SQL-92 (major revision) SQL-99 (major extensions in OO, current standard)

SQL Language

Data Definition Language (DDL) Create tables, specifying the columns and their types of columns,

the primary keys, etc. Drop tables, add/drop columns, add/drop constraints – primary

key, unique, etc.

Data Manipulation Language (DML) Update, Insert, Delete tuples Query the data

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SQL DDL

DDL allows the specification of:

The schema for each relation

The type and domain of values associated with each attribute

Integrity constraints Will see many types of these constraints

The set of indices to be maintained for each relation

Security and authorization information for each relation

The physical storage structure of each relation on disk

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DDL: Creating TablesCREATE TABLE <tableName> (

<col> <type>,<col> <type>,…<col> <type>,[CONSTRAINT <cName>] PRIMARY KEY (…),[CONSTRAINT <cName>] UNIQUE (…),[CONSTRAINT <cName>] FOREIGN KEY (…)

REFERENCES <tableName> (…));

Domain Types in SQL String:

string: variable size string with no maximum (theoretically ) char(n): Fixed length character string, with user-specified length n varchar(n): Variable length character strings, with user-specified maximum length n

Numbers: Int: Integer Smallint (short): Small integer float(n): Floating point number, with user-specified precision of at least n digits. Decimal(n,d): Double-precision of n decimal digits with decimal point at position d

Bits: Bit(n): String of bits of fixed size n (user-defined) Bit Varying(n): String of variable-size bits of maximum n (user-defined)

Boolean: Boolean type, True or False

Date & Time: Date: Date field with certain format Time: Date plus time with a certain format

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customer (name: string, DoB: Date, Phone: Char(10), …)customer (name: string, DoB: Date, Phone: Char(10), …)

DDL ---- Creating Relations

Create “Students” relation

CREATE TABLE Students(sid: CHAR(20), name: CHAR(20), login: CHAR(10), age: INTEGER, gpa: REAL);

CREATE TABLE Enrolled(sid: CHAR(20), cid: Varchar(20),

enrollDate: date, grade: CHAR(2))

Observe that type (domain) of each field is specified

Types are enforced by DBMS whenever tuples are added or modified

Create “Enrolled” relation

CREATE TABLE Courses(cid: Varchar(20), name: string, maxCredits : integer,

graduateFlag: boolean);

Create “Courses” relation

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Dropping & Altering Tables Dropping (Deleting) a table

DROP TABLE <tableName>;

Altering a table to add a column ALTER TABLE <tableName> ADD <col> <type>; The initial value of the column is Null

Altering a table to drop a column ALTER TABLE <tableName> DROP COLUMN <col>; Some DBMSs do not support dropping columns

Integrity Constraints (ICs)

IC: condition that must be True for any instance of the database

ICs are specified when schema is defined ICs are checked when relations are modified

A legal instance of a relation is one that satisfies all specified ICs.

DBMS should not allow illegal instances.

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IC: Key Constraints Two types of key constraints can be defined

Primary Key Unique

The other candidate keys

Each relation may have only one primary key but, possibly many unique keys

Primary key attributes do not accept Null

Create “Students” relation

CREATE TABLE Students(sid: CHAR(20) Primary Key, name: CHAR(20), login: CHAR(10) Unique, age: INTEGER, gpa: REAL);

CREATE TABLE Courses (cid: Varchar(20) Primary Key, name: string, maxCredits : integer, graduateFlag: boolean);

Create “Courses” relation

IC: Key Constraints (Cont’d)

Another way to define Keys

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Create “Students” relation

CREATE TABLE Students(sid: CHAR(20), name: CHAR(20), login: CHAR(10), age: INTEGER, gpa: REAL,

Primary Key (sid), Unique (login));

CREATE TABLE Courses (cid: Varchar(20), name: string, maxCredits : integer, graduateFlag: boolean, Primary Key (cid));

Create “Courses” relation

CREATE TABLE Enrolled(sid: CHAR(20), cid: Varchar(20), enrollDate: date, grade: CHAR(2),

Primary Key (sid, cid, enrollDate));

Create “Enrolled” relation

Violations of Key Constraints

DBMS prevents this from happening

Example customer (name: string, DoB: Date, Phone: Char(10), address: string);

(“Mike”, “10/25/1988”, “1765432123”, “NY”); Inserted

(“Mike”, “5/10/1990”, “1999887665”, “CA”); Raise an error

(null , “5/10/1990”, “1999887665”, “CA”); Raise an error

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Violation of key constraints

Primary key violation if: There is a row with null values for any attribute of primary key, OR Two rows with same values for all attributes of primary key

Unique key violation if: Two rows have the same non-null values in the unique attributes Null values are not equal to each other

DBMS will prevent this from happening

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Adding/Dropping Constraints

Adding a constraint (after creating the table) ALTER TABLE <tableName> ADD CONSTRAINT

<cName> <constraint type> …

Dropping a constraint ALTER TABLE <tableName> DROP CONSTRAINT

<cName>

Constraints Limits the Data

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CREATE TABLE Enrolled(sid: CHAR(20), cid: Varchar(20), enrollDate: date, grade: CHAR(2),

Primary Key (sid, cid));

Create “Enrolled” relation

CREATE TABLE Enrolled(sid: CHAR(20), cid: Varchar(20), enrollDate: date, grade: CHAR(2),

Primary Key (sid, cid, enrollDate));

Create “Enrolled” relation

A student can take a course only once.

A student can take a course many times on different dates.

Other Constraint Types

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Create “Students” relation

CREATE TABLE Students(sid: CHAR(20), name: CHAR(20) NOT NULL, login: CHAR(10), age: INTEGER, gpa: REAL Default 0.0,

Primary Key (sid), Unique (login));

CREATE TABLE Courses (cid: Varchar(20), name: string, maxCredits : integer, graduateFlag: boolean Default False, Primary Key (cid));

Create “Courses” relation

NOT NULL & DEFAULT constraints

Other Constraint Types (Cont’d)

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CREATE TABLE Enrolled(sid: CHAR(20), cid: Varchar(20), enrollDate: date, grade: CHAR(2),

Primary Key (sid, cid, enrollDate),Constraint gradeVal check grade in (‘A+’, ‘A-’, ‘B+’, ‘B-’));

Create “Enrolled” relation

Domain constraint

IC: Foreign Key Constraints(Referential Integrity )

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Create “Students” relation

CREATE TABLE Students(sid: CHAR(20), name: CHAR(20), login: CHAR(10), age: INTEGER, gpa: REAL);

CREATE TABLE Courses (cid: Varchar(20), name: string, maxCredits : integer, graduateFlag: boolean);

Create “Courses” relation

CREATE TABLE Enrolled(sid: CHAR(20), cid: Varchar(20), enrollDate: date, grade: CHAR(2));

Create “Enrolled” relationForeign key

Foreign key

Foreign Keys, Referential Integrity

Foreign key : Set of fields in one relation that "refer" to a tuple in another relation (like a pointer)

Foreign key : FK in referencing relation must match PK of referenced relation. Match = same number of columns, compatible data types (column names

can be different) The relationship is many-to-one from the “referencing” to the “referenced”

sid name login age gpa

53666 Jones jones@cs 18 3.453688 Smith smith@eecs 18 3.253650 Smith smith@math 19 3.8

sid cid grade53666 Carnatic101 C53666 Reggae203 B53650 Topology112 A53666 History105 B

Enrolled (referencing relation)Students (referenced relation)

Foreign Key Primary Key

Foreign Keys in SQL

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Create “Students” relation

CREATE TABLE Students(sid: CHAR(20) Primary Key, name: CHAR(20), login: CHAR(10), age: INTEGER, gpa: REAL);

CREATE TABLE Courses (cid: Varchar(20) Primary Key, name: string, maxCredits : integer, graduateFlag: boolean);

Create “Courses” relation

CREATE TABLE Enrolled(sid: CHAR(20) Foreign Key References

(Students.sid), cid: Varchar(20), enrollDate: date, grade: CHAR(2),

Constraints fk_cid Foreign Key cid References (Courses.cid));

Create “Enrolled” relation

Alter Table Enrolled Add Constraints fk_cid Foreign Key cid References Courses(cid));

sid & cid have to be already defined as

primary keys

sid & cid have to be already defined as

primary keys

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Violation of Foreign Key Constraints

Suppose we have: FOREIGN KEY R1 (S1) REFERENCES R2 (S2)

This constraint is violated if: If there is a value in R1.S1 which does not exist in R2.S2

The DBMS prevents this from happening

Enforcing Referential Integrity

Consider Students and Enrolled; sid in Enrolled is a foreign key that references Students.

What if inserting in “Enrolled” values (“4545”, “History105”, “A”) DBMS will reject it because no student with sid “4545”

Enforcing Referential Integrity (Cont’d)

Deletion: What if an Enrolled tuple is deleted? No problem. It is allowed

Enforcing Referential Integrity (Cont’d)

Deletion: What if a Student tuple is deleted? Cascading -- Also delete all Enrolled tuples that refer to it

No Action -- Disallow deletion of a Students tuple since there are dependent tuples

Set Default -- Set sid in Enrolled tuples that refer to it to a default sid

Set Null -- Set sid in Enrolled tuples that refer to it to a special value null, denoting `unknown’ (Not always applicable)

Same rules apply when updating Students.sid

The two most common actions are: Cascading or No Action

Referential Integrity in SQL

SQL/99 supports all 4 options on deletes & updates:

CREATE TABLE Enrolled(sid: CHAR(20) Foreign Key References

(Students.sid), cid: Varchar(20), enrollDate: date, grade: CHAR(2),

Constraints fk_cid Foreign Key cid References (Courses.cid) ON DELETE CASCADE ON UPDATE NO ACTION);

Cyclic Dependencies

What if cyclic dependencies between two tables exists? Table R has a foreign key to Table S (Say R.a references S.b) Table S has a foreign key to Table R (Say S.c references R.d)

At insertion time: When insert into R will be rejected because S.b does not yet exits When insert into S will be rejected because R.d does not yet exits

Solution (Many) – The easiest is: Disable the constraint on one table, say S Insert the tuple into S, then insert into R Enable the constraint on S again

Alter Table S Disable Constraint <name>;

Alter Table S Enable Constraint <name>;

Where do ICs Come From? ICs are based upon semantics of real-world enterprise being

described in database relations.

We can check a database instance to see if an IC is violated ? In typical cases, DBMS should make sure all constraints are enforced

Given an instance, can we tell what are the ICs? No, NEVER ! An IC is a statement about all possible instances! They need to be checked from the schema definition not from a

given instance

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Relational Model: Summary

Structure Relations (Tables) Attributes (Columns, Fields)

Constraints + Constraint Enforcement Domain, Default, and Not Null Constraint Key Constraint

Primary key, candidate key (unique) Foreign Key Constraint

Inserting, Updating, Deleting Data This is performed using Data Manipulation

Language of SQL (DML)

Insertion Insert into Students values (“1111”, …);

Deletion Delete from Students; Delete from Students Where sid = “1111”;

Update Update Students Set GPA = GPA + 0.4; Update Students Set GPA = GPA + 0.4 Where sid = “1111”;

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