The document discusses entity relationship diagrams (ERD), subqueries, and views. It covers concepts such as weak entities, derived attributes, multi-valued attributes, composite attributes, and examples of how to represent these in an ERD. It also provides examples of subqueries used in SELECT statements and how to create views in SQL along with exercises for practicing views.

1. ERD & Subquery & View
April 2017
Business Economics & Information Technology

2. OUTLINE
 weak entity
 Derived attribute
 Multi-valued attribute
 Composite attribute

3. WEAK ENTITY
◼A weak entity is an entity that depends on the existence of another entity and cannot be uniquely identified
by its attributes
◼A weak entity is represented by double rectangles
◼A weak uses a foreign key combined with its attributed to form the primary key.
Example:
The order item will be meaningless without an order so it depends on the existence of order
.
What is the priamry key of
Order-Item entity?

4. MORE EXAMPLE OF WEAK ENTITES
Depends
ID
DEP_ID DEP_NAME DEP_DOB
20 Lisbeth 22-jul-1985
30 Rasmus 30-OCT-1990
40 Morten NULL
EMP_ID EMP_NAME EMP_DOB
100 ALLEN 17-DEC-1955
101 SARA 06-APRIL-1959
102 JOHN 22-FEB-1990
EMP_ID
100
100
101
What is the priamry key of
Dependent entity?
What is the cardinality
between tables?

5. DISCRIMINATOR
◼The primary key of a weak entity is a composite key formed from the primary key of the entity and partial key
of the weak entity.
◼Partial key is called discriminator
◼Payment No Is a discriminator of the payment entity
What is the discrimiator of the dependent
entity?

6. DERIVED ATTRIBUTE
◼A derived attribute is an attribute whose value is calculated (derived) from other
attributes.
◼The derived attribute need not be physically stored within the database
◼A derived attribute is represented by dashed ellipses
Example:
An employee’s age may be found by computing the integer value of the difference between the current date
and the birth date of the employee
Mention derived attribute of the Case 5 (Danish Consumer Council)
Employee

7. Multivalued Attribute
◼A multi-valued attribute has more than one value for a particular entity
◼Multivalued attribute is represented by double ellipse
Example:
Phone is a multivalued attribute because it is possible for each person to have more than
employee has more than one phone number.
 Skill is a multivalued attribute for EMPLOYEE entity
Is a table/relation with multivalued
attribute in the first Normal form?

8. Transform of a multivalued attribute
◼A multivalued attribute can be converted to a weak entity
Example:
What are the primary key of Phone entity?
PERSON_ID
FIRST_NAME
LAST_NAME
PHONE
PERSON_ID
FIRST_NAME
LAST_NAME
PHONE NO
PHONE TYPE
PersonPerson
Phone
1 M

9. COMPOSITE ATTRIBUTE
◼An attribute is considered composite if it comprises two or more other attributes.
Examples
◼Composite attributes are represented by ellipses that are connected with an ellipse (Tree
structure)

10. Exercise
Identify composite, multivalued, and derived attributes for Customer entity.

11. SUBQUERY
◼A subquery/Inner query/Nested query is a SQL query within another SQL query and embedded within the
WHERE clause.
◼Subqueries are most frequently used with the SELECT statement. The basic syntax is as follows:
SELECT column_name FROM table
WHERE column_name OPERATOR
(SELECT column_name FROM
table
[WHERE] )

12. Example
list the first name, last name, salary of the employees, paid more than 'Alexander’
First_name Last_name salary
John Knudsen 12000
Sara Alexander 9000
Lisbeth Nielsen 8000
Employees

13. EXAMPLE
List of the employees that paid less than ’Randy’

14. EXAMPLE
Drop database if exists sub_query;
create database sub_query;
Create table employee_SUB (
ID INT NOT NULL PRIMARY KEY,
FirstName varchar(20),
LastName varchar(20),
dept varchar(10),
salary FLOAT(8,2)
);
insert into employee_SUB values(100,'Thomas', 'Larsen','Sales',5000.98);
insert into employee_SUB values(200,'Jason', 'Johansen','Technology',5500.99);
insert into employee_SUB values(300,'Mayla', 'Nielsen','Technology',7000);
insert into employee_SUB values(400,'Nisha', 'Evanovic','Marketing',9500);
insert into employee_SUB values(500,'Randy', 'Pedersen','Technology',6897.55);

15. EXAMPLE
SELECT * FROM employee_SUB
WHERE
SALARY <
(SELECT SALARY FROM employee_SUB
WHERE
FirstName='Randy');

16. EXERCISE
List of the employees that paid less than ’Randy’ and more than ‘Thomas’

17. EXERCISES
List name of faculty with a salary less than the average salary
List of faculty at Technology Departmen with a salary less than the average salary

18. Logical representations (Subset of data from one or more tables or views)
◼Table: Physical unit
◼View: Logical representation
View
ViewTable

19. WHY USE A VIEW?
Restrict database access.
Make complex queries easy.
Represent data from different tables.
Represent different Views of the same data.

20. Creating a View
CREATE [OR REPLACE] VIEW //REPLACE if view already exists
VIEW_NAME
AS
SELECT COLUMN1, COLUMN2,…
FROM TABLE_NAME
WHERE CONDITION;
 EXAMPLE 1
 EXAMPLE 2
CREATE VIEW [List of Chair] AS
SELECT ProductID, ProductName, productprice
FROM Products
WHERE productName = ‘Chair’;
CREATE VIEW [List of products more than 1000] AS
SELECT ProductID, ProductName
FROM Products
WHERE productprice > 1000’;

21. Example
Create a view called VIEW_DEPT_TECH with Firstname, salary, dept for employees in
Technology department only (Use employee_SUB table)
CREATE VIEW VIEW_DEPT_TECH Name of the view
AS
SELECT Firstname, salary,dept
FROM employee_SUB
WHERE dept='Technology';
Display the data from VIEW_DEPT_TECH view.
SELECT * FROM VIEW_DEPT_TECH;

22. Create a view called VIEW_SALARY with minimum, maximum, and average of salary
Display the data from VIEW_SALARY view.
Exercises

23. Exercises
Create a view called VIEW_SALARY_LESS_AVERAGE with name and salary of faculty that have
a salary less than the average salary
Display the data from VIEW_SALARY_LESS_AVERAGE

24. drop database if exists threetables;
create database threetables;
use threetables;
create table Customers
(
customerID int not null,
customerName varchar(50),
customerLastName varchar(50),
primary key (customerID)
);
create table Products
(
productID int not null,
productName varchar(100),
productPrice float,
primary key (productID)
);
create table Orders
(
OrderID int not null primary key,
customerID int not null,
productID int not null,
Quantity INT,
OrderDate Date,
FOREIGN KEY (customerID) REFERENCES Customers(customerID),
FOREIGN KEY (productID) REFERENCES Products(productID)
);
Exercises

25. Exercises
insert into Customers values (1,'Steen','Andersen');
insert into Customers values(2,'Helle','Rasmusen');
insert into Customers values(3,'Mette','Nilsen');
insert into Products values(1,'Samsung', 5000.0);
insert into Products values(2,'Apple', 4500.0);
insert into Products values(3,'Asus', 5500.0);
◼ insert into Orders values (1,1,1,3,current_date());
◼ insert into Orders values (2,1,2,5,current_date());
◼ insert into Orders values (3,2,2,1,current_date());
◼ insert into Orders values (4,1,3,2,'2015-02-02');
◼ insert into Orders values (5,2,2,6,'2016-03-02');

26. Exercises
Create a view called TOTAL with attributes cutomerID, ProductID, ProductPrice, Quantity and
Total amount per order
Display the data from TOTAL

27. Exercises
Create a view called TOTAL_2017 with attributes cutomerName, ProductName,
OrderID, ProductPrice, Quantity and Total amount per order for 2017
Display the data from TOTAL_2017

28. READ ONLY OPTIONS
CREATE VIEW PRODUCTS_VIEW
AS
SELECT productName, productPrice FROM Products where productName= 'Apple'
WITH READ ONLY;
//Data may not be modified

29. Removing a View
DROP VIEW TOTAL;

30. HAND-IN
30
NAME
CPR
ADDRESS
ZIP_CODE
PHONE
BIRTH_OF_DATE
GENDER
RACE
SMOKER
DRUG_ABUSE
DRIVINGLICENSNUM
DRIVINGLICENSESTATE
NEAREST RELATIVE
PHONE NUMBER OF NEAREST RELATIVE
INSURANCE NUMBER
INSURANCE NAME
DATE OF INSURANCE COVERAGE
REASON OF HOSPITALIZATION
DATE OF HOSPITALIZATION
Draw an ERD with the following information. Identify entitets, weak entities, derived attribute, multivalues
attribute, composite attribute, keys and the cardinalities between the tables. Implements the tables in MYSQL.
Medicine used
Dosage of Medicine
Date medicine started
Date medicine end