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Instructor: Mohamed Eltabakh

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1 Instructor: Mohamed Eltabakh meltabakh@cs.wpi.edu
The Relational Model Instructor: Mohamed Eltabakh

2 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

3 Design and Build Phases
ER Model & ERD Build the database Relational Model

4 Relational Model Structure: Relations (also called Tables)
Attributes (also called Columns or Fields)

5 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)

6 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)

7 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

8 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 Arity = 3 Cardinality = 4

9 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 Student Student Student sNumber sName 1 Dave 2 Greg sNumber sName 2 Greg 1 Dave sName SNumber Greg 2 Dave 1 These relations are equivalent

10 Keys of Relations Super Keys Candidate Keys Primary Keys Set vs. Bag
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 customer (customer_name, customer_street, customer_city) Assuming the customer name is unique

11 To create these relations (tables) We use SQL

12 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)

13 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

14 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

15 DDL: Creating Tables CREATE TABLE <tableName> (
<col> <type>, [CONSTRAINT <cName>] PRIMARY KEY (…), [CONSTRAINT <cName>] UNIQUE (…), [CONSTRAINT <cName>] FOREIGN KEY (…) REFERENCES <tableName> (…) );

16 DDL ---- Creating Relations
Create “Students” relation Create “Courses” relation CREATE TABLE Students (sid: CHAR(20), name: CHAR(20), login: CHAR(10), age: INTEGER, gpa: REAL); CREATE TABLE Courses (cid: Varchar2(20), name: Varchar2(50), maxCredits : integer, graduateFlag: char(1)); Create “Enrolled” relation Observe that type (domain) of each field is specified Types are enforced by DBMS whenever tuples are added or modified CREATE TABLE Enrolled (sid: CHAR(20), cid: Varchar2(20), enrollDate: date, grade: CHAR(2)) 15

17 Domain Types in SQL String Data: Numbers: Boolean: Date & Time:
char(n): Fixed length character string, with user-specified length n varchar2(n): Variable length character strings, with user-specified maximum length n Numbers: int: Integer Real: floating point number Number(p,s): Number with precision p and scale s Boolean: Make it char(1) [Y/N or T/F], or number(1) [1/0] Date & Time: Date: Date field with certain format Timestamp: Date plus time with a certain format customer (name: varchar2(100), DoB: Date, Phone: Char(10), …)

18 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

19 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. Examples: Student ID must be unique The combination of first + middle + last names must be unique The grade must be one of ( A, B, C, D) The gender must be (M, F) Account balance must be >= 0

20 IC: Key Constraints (sid: CHAR(20) Primary Key, name: CHAR(20),
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, Unique attributes accept Null Create “Students” relation Create “Courses” relation CREATE TABLE Students (sid: CHAR(20) Primary Key, name: CHAR(20), login: CHAR(10) Unique, age: INTEGER, gpa: REAL); CREATE TABLE Courses (cid: Varchar2(20) Primary Key, name: varchar2(50), maxCredits : integer, graduateFlag: char(1));

21 IC: Key Constraints (Cont’d)
Another way to define Keys CREATE TABLE Students (sid: CHAR(20), name: CHAR(20), login: CHAR(10), age: INTEGER, gpa: REAL, Constraint pk_sid Primary Key (sid), Constraint u_login Unique (login)); Create “Students” relation A relation can have only one primary key, but many unique keys CREATE TABLE Enrolled (sid: CHAR(20), cid: Varchar(20), enrollDate: date, grade: CHAR(2), Constraint pk_cols Primary Key (sid, cid, enrollDate)); Create “Enrolled” relation That is how you define constraints on multiple columns

22 Violations of Key Constraints
DBMS prevents this from happening Example customer (name: varchar2(50), Phone: Char(10), address: varchar2(100)); (“Mike”, “ ”, “NY”); Inserted (“Mike”, “ ”, “CA”); Raises an error (duplication) (null , “ ”, “CA”); Raises an error (null value)

23 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

24 Adding/Dropping Constraints
Can be done after creating tables Adding a constraint ALTER TABLE <tableName> ADD CONSTRAINT <cName> <constraint type> … Dropping a constraint ALTER TABLE <tableName> DROP CONSTRAINT <cName>

25 Example Then add constraints… (sid: CHAR(20),
CREATE TABLE Students (sid: CHAR(20), name: CHAR(20), login: CHAR(10), age: INTEGER, gpa: REAL); Create “Students” relation with no constraints Alter table Students add constraint StudentPrimaryKey Primary Key (sid); Alter table Students add constraint StudentUnique Unique (login); Then add constraints…

26 Constraints Limits the Data
Create “Enrolled” relation CREATE TABLE Enrolled (sid: CHAR(20), cid: Varchar2(20), enrollDate: date, grade: CHAR(2), Constraint pk Primary Key (sid, cid, enrollDate)); Create “Enrolled” relation A student can take a course many times on different dates. CREATE TABLE Enrolled (sid: CHAR(20), cid: Varchar2(20), enrollDate: date, grade: CHAR(2), Constraint pk Primary Key (sid, cid)); A student can take a course only once.

27 Not Null & Default Constraint Types
Create “Students” relation Create “Courses” relation CREATE TABLE Students (sid: CHAR(20), name: CHAR(20) NOT NULL, login: CHAR(10), age: INTEGER, gpa: REAL Default 0, Constraint pk Primary Key (sid), Constraint u1 Unique (login)); CREATE TABLE Courses (cid: Varchar2(20), name: varchar2(50), maxCredits : integer, graduateFlag: char(1) Default ‘Y’); NOT NULL & DEFAULT constraints

28 Domain Values Constraint
Create “Enrolled” relation CREATE TABLE Enrolled (sid: CHAR(20), cid: Varchar2(20), enrollDate: date, grade: CHAR(2), Constraint pk Primary Key (sid, cid, enrollDate), Constraint gradeVal check (grade in (‘A+’, ‘A-’, ‘B+’, ‘B-’)) ); Domain constraint

29 IC: Foreign Key Constraints (Referential Integrity )
Create “Students” relation Create “Courses” relation CREATE TABLE Students (sid: CHAR(20), name: CHAR(20), login: CHAR(10), age: INTEGER, gpa: REAL); CREATE TABLE Courses (cid: Varchar2(20), name: varchar2(50), maxCredits : integer, graduateFlag: char(1)); Foreign key Foreign key Create “Enrolled” relation CREATE TABLE Enrolled (sid: CHAR(20), cid: Varchar2(20), enrollDate: date, grade: CHAR(2));

30 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” Enrolled (referencing relation) Students (referenced relation) Foreign Key Primary Key 7

31 Foreign Keys in SQL Create “Students” relation
Create “Courses” relation CREATE TABLE Students (sid: CHAR(20) Primary Key, name: CHAR(20), login: CHAR(10), age: INTEGER, gpa: REAL); CREATE TABLE Courses (cid: Varchar2(20) Primary Key, name: varchar2(50), maxCredits : integer, graduateFlag: char(1)); Create “Enrolled” relation CREATE TABLE Enrolled (sid: CHAR(20) Foreign Key References Students (sid), X: Varchar2(20), enrollDate: date, grade: CHAR(2), Constraint fk_cid Foreign Key (X) References Courses (cid)); Two ways to define the FK constrain while creating a table Alter Table Enrolled Add Constraints fk_cid Foreign Key X References Courses(cid)); That is a third way

32 sid & cid have to be already defined as primary keys
Foreign Keys in SQL Create “Students” relation Create “Courses” relation CREATE TABLE Students (sid: CHAR(20) Primary Key, name: CHAR(20), login: CHAR(10), age: INTEGER, gpa: REAL); CREATE TABLE Courses (cid: Varchar2(20) Primary Key, name: varchar2(50), maxCredits : integer, graduateFlag: char(1)); Create “Enrolled” relation sid & cid have to be already defined as primary keys CREATE TABLE Enrolled (sid: CHAR(20) Foreign Key References Students (sid), X: Varchar2(20), enrollDate: date, grade: CHAR(2), Constraint fk_cid Foreign Key (X) References Courses (cid)); Alter Table Enrolled Add Constraints fk_cid Foreign Key X References Courses(cid));

33 Violation of Foreign Key Constraints
Enrolled (referencing relation) Students (referenced relation) Foreign Key Primary Key Will be rejected DBMS will prevent foreign key values that does not exist in the primary key column(s)

34 Enforcing Referential Integrity
Deletion: What if an Enrolled tuple is deleted? No problem. It is allowed 13

35 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 13

36 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: Varchar2(20), enrollDate: date, grade: CHAR(2), Constraints fk_cid Foreign Key cid References Courses (cid) ON DELETE CASCADE ON UPDATE NO ACTION); 14

37 Cyclic Dependencies At insertion time: Another solution
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) – One option: Disable the constraint on one table, say S Insert the tuple into S, then insert into R Enable the constraint on S again Another solution Group the insertions in one transaction Begin Transaction <the inserts> End Transaction Alter Table S Disable Constraint <name>; Alter Table S Enable Constraint <name>; Constraints are checked at the end of the transaction

38 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

39 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 SQL DDL commands

40 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 Where sid = ‘1111’;

41 Advise…. Google is your best and fastest friend to get the SQL syntax you want… Make sure to search for “Oracle SQL ….” E.g., how a foreign key can consists of multiple columns


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