SQL for Data Retrieval

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Running Example

Data Preparation

• Login to SQL server using your account• Download three SQL script files from wiki page and

open them in SQL Server Management Studio.– SQL-Delete-Tables.sql– SQL-Create-Tables.sql– SQL-Insert-Data.sql

• Select your database– Your database name is your PSU ID– Alternatively, you can add one statement at the first line

of each script: USE [your PSU ID];

• Execute scripts.

Review of Previous Class: Queries

• SELECT is the best known SQL statement• SELECT will retrieve information from the

database that matches the specified criteria using the SELECT / FROM / WHERE framework

SELECT ColumnName

FROM TableWHERE Condition;

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Exercise 1: Review Questions

• Q1. Show the phone number of Accounting department (use Department table)– Use DEPARTMENT table– Use “WHERE”, slide #13 (week4-3.pptx)

• Q2. Show employees with FirstName as ‘Mary’ and LastName starting with letter ‘A’– Use “OR”, slide #16– Use wildcard, slide #21

SQL for Data Retrieval:Sorting the Results

• Query results may be sorted using the ORDER BY clause

SELECT *FROM EMPLOYEEORDER BY LastName;

SQL for Data Retrieval:Sorting the Results

• By default, SQL sorts in ascending order. • To sort in descending order, use:

SELECT FirstName, LastName, Phone, DepartmentFROM EMPLOYEEORDER BY Department DESC;

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SQL for Data Retrieval:Sorting the Results

• Query FirstName, LastName, Phone, Department from EMPLOYEE table, sort the result by Department in descending order, and then by LastName in ascending order

SELECT FirstName, LastName, Phone, DepartmentFROM EMPLOYEEORDER BY Department DESC, LastName ASC;

SQL for Data Retrieval:Built-in SQL Functions

• SQL provides several built-in functions – COUNT

• Counts the number of rows that match the specified criteria– MIN

• Finds the minimum value for a specific column for those rows matching the criteria

– MAX• Finds the maximum value for a specific column for those rows matching

the criteria– SUM

• Calculates the sum for a specific column for those rows matching the criteria

– AVG• Calculates the numerical average of a specific column for those rows

matching the criteria

SQL for Data Retrieval:Built-in Function Examples

• Count the total number of employees:

SELECT COUNT(*)FROM EMPLOYEE;

• Find the minimum, maximum, and average hours for all projects.

SELECT MIN(MaxHours) AS MinimumHours,MAX(MaxHours) AS MaximumHours,AVG(MaxHours) AS AverageHours

FROM PROJECT

SQL for Data Retrieval:Built-in Function Examples

• SQL standard does not allow column names to be mixed with built-in functions, except in certain uses of GROUP BY clause (to be discussed next)

SELECT MaxHours, SUM(MaxHours)FROM PROJECTWHERE ProjectID <= 1200;

Bad query. MaxHours is not an aggregate result, but SUM() is.

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Exercise 2

• Q1. Count how many projects with MaxHours less than 130– Use “WHERE”

• Q2. Show average MaxHours of all projects

• Q3. (Challenging) Count how many projects with MaxHours less than the average MaxHours?

SQL for Data Retrieval:Providing Subtotals: GROUP BY

• Subtotals may be calculated by using the GROUP BY clause– Show how many employees in each department

SELECT Department,COUNT(*) AS NumOfEmployees

FROM EMPLOYEEGROUP BY Department

• The HAVING clause may be used to restrict which data is displayed– Only show those departments with more than one

employeeSELECT Department,

COUNT(*) AS NumOfEmployeesFROM EMPLOYEEGROUP BY Department

HAVING COUNT(*) > 1;

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Exercise 3

• Q1. Group assignments by employees. Show employee number and total hours worked

• Q2. Add a constraint to Q1: only show the employees with total hours worked more than 100

• Q3. Add a constraint to Q1: only show the employees with employee number less than 5– Try to use two ways to answer this query: (1) Use

“WHERE”, (2) Use “HAVING”

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

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

18Q2. 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, LastNameFROM 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);

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

SELECT FirstName, LastNameFROM EMPLOYEEWHERE EmployeeNumber IN

(SELECT EmployeeNumber FROM ASSIGNMENT WHERE HoursWorked < 20);

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Example 1: Use Join

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

AND HoursWorked < 20;

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

Shared column: EmployeeNumber

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Exercise 4: Use Join

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

Shared column: EmployeeNumber