sql for data retrieval. review questions of previous class q1. show the sum of hours worked for...

SQL for Data Retrieval

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Review Questions of Previous Class

• Q1. Show the sum of hours worked for project with ID 1200 (use ASSIGNMENT table)– Use “SUM” and “WHERE” slide #9 (3-SQL-4.pptx)

• Q2. Count how many distinct departments in project table– Use “COUNT” slide #9 (3-SQL-4.pptx)– Use “DISTINCT”

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Review Questions of Previous Class

• Q3. Group projects by departments. Show department names and number of projects associated with each department– Use “group by”, slide #13 (3-SQL-4.pptx)

• Q4. Add a constraint to the question above. Only show the departments with number of projects more than 1– Use ”group by” and “having”, slide #13 (3-SQL-4.pptx)

Retrieving Information from Multiple Tables

• Two approaches– Subqueries– Joins

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Example 1: Querying Two Tables• Show the names of employees who worked less than 20 hours

Q2. Get names of the employees with employee number 4, 5: Tom Caruthers, Heather Jones

Q1. Check “worked less than 20 hours” in ASSIGNMENT table: employee number 4, 5

EMPLOYEE

ASSIGNMENT

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Example 1: Use Subquery

SELECT EmployeeNumberFROM ASSIGNMENTWHERE HoursWorked < 20;

SELECT FirstName, LastNameFROM EMPLOYEEWHERE EmployeeNumber IN (4, 5);

SELECT FirstName, LastNameFROM EMPLOYEEWHERE EmployeeNumber IN

(SELECT EmployeeNumber FROM ASSIGNMENT WHERE HoursWorked < 20);

• Show the names of employees who worked less than 20 hours

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Exercise 1: Use Subquery• Show the names of employees who is assigned to ProjectID 1000

– First write two separate queries and then merge into one query

7Step 2. Show the names of employees with the employee numbers from Q1

Q1. Find employee numbers who is assigned to ProjectID 1000

EMPLOYEE

ASSIGNMENT

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Example 1: Use Join

SELECT FirstName, LastName, HoursWorkedFROM EMPLOYEE AS E, ASSIGNMENT AS AWHERE E.EmployeeNumber = A.EmployeeNumber

AND HoursWorked < 20;

SELECT EmployeeNumberFROM ASSIGNMENTWHERE HoursWorked < 20;

SELECT FirstName, LastNameFROM EMPLOYEEWHERE EmployeeNumber IN (4, 5);



(SELECT EmployeeNumber FROM ASSIGNMENT WHERE HoursWorked < 20);

Show columns from multiple tables

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Example 1: Use Join

SELECT FirstName, LastName, HoursWorkedFROM EMPLOYEE AS E, ASSIGNMENT AS AWHERE E.EmployeeNumber = A.EmployeeNumber

AND HoursWorked < 20;


Shared column: EmployeeNumber

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Exercise 1: Use Join

• Show the names of employees who is assigned to ProjectID 1000


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Exercise 2: Use Subquery

• Show the project names assigned to EmployeeNumber 4

ASSIGNMENT

PROJECT

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• Show the project names assigned to EmployeeNumber 4

Shared column: ProjectID

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Example 2: Querying Three Tables

• Show the names of employees who are assigned with projects associated with Finance department

Names of employees


Associated with Finance department

Shared column: ProjectID

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Example 2: Use Subquery • Show the names of employees who are assigned

with projects associated with Finance department

ASSIGNMENT

PROJECT

Q1. Associated with Finance department: 1100, 1400

Q3. Employee names with number in {4,6,4,5,6}

EMPLOYEE

Q2. EmployeeNumber assigned to project 1100 or 1400: 4,6,4,5,6

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Example 2: Use Subquery

SELECT EmployeeNumberFROM ASSIGNMENTWHERE ProjectID IN (1100, 1400);

SELECT FirstName, LastNameFROM EMPLOYEEWHERE EmployeeNumber IN (4, 5, 6);


(SELECT EmployeeNumber FROM ASSIGNMENT

WHERE ProjectID IN(SELECT

ProjectIDFROM PROJECTWHERE

Department = 'Finance')

);


SELECT ProjectIDFROM PROJECTWHERE Department = 'Finance';

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Example 2: Use Join

SELECT DISTINCT FirstName, LastNameFROM EMPLOYEE AS E, PROJECT AS P, ASSIGNMENT AS AWHERE E.EmployeeNumber = A.EmployeeNumber AND P.ProjectID = A.ProjectID AND P.Department = 'Finance';


Shared column: ProjectIDShared column: EmployeeNumber

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Exercise 3: Use Subquery

• Show all the project names that are assigned to Ken– Write 3 separate queries first and then merge them into 1

ASSIGNMENT

PROJECT

Q3. Names of the projects with ID in (1000, 1300)

Q1. Employee numbers with first name as Ken: 10

EMPLOYEE

Q2. ProjectID assigned to employee number 10: 1000, 1300

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• Show all the project names that are assigned to Ken– Use join

Shared column: ProjectIDShared column: EmployeeNumber

More about Modifying Data

• Insert– Add a new row in a table

• Update– Update the data in a table that matches the

specified criteria• Delete– Delete the data in a table that matches the

specified criteria

Changing Data Values: UPDATE

• To change the data values in an existing row (or set of rows) use the Update statement

UPDATE EMPLOYEESET Phone='360-555-1234'WHERE EmployeeNumber = 11;

UPDATE PROJECTSET ProjectID = 2000WHERE ProjectID = 1000

UPDATE DEPARTMENTSET DepartmentName = 'Public Relation'WHERE ProjectID = 'Marketing'

Deleting Data: DELETE

• To delete a row or set of rows from a table using the DELETE statement

DELETE FROM EMPLOYEEWHERE EemplyeeNumber = 12;

DELETE FROM EMPLOYEEWHERE EemplyeeNumber = 8;

Update/Delete Rules

• CASCADE– Affect the primary key as well as the foreign keys

• NO ACTION– Cannot be changed/deleted unless a record is NOT

referred by any other table at all.

sql for data retrieval. review questions of previous class q1. show the sum of hours worked for...

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