Structured Query LanguagePart I

Chapter ThreeCIS 218

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Structured Query Language

• Structured Query Language– Acronym: SQL– Pronounced as “S-Q-L” [“Ess-Que-El”]– Also pronounced as “Sequel”

• Originally developed by IBM in 1970s– SEQUEL language

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SQL Defined

• SQL is not a programming language• SQL is comprised of:

– A data definition language (DDL)• Used to define structures• Tables, columns, etc.

– A data manipulation language (DML)• Used to create, add and delete data rows• Used to retrieve (query) data

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SQL Queries

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

database that matches specified criteria• Uses SELECT/FROM/WHERE syntax

SELECT col1, col2, …

FROM tableName

WHERE criteria;

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Displaying All Columns

• To show all of the column values for the rows that match the specified criteria, use an asterisk ( * )

SELECT *

FROM Tech

• Returns all columns and all rows

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A Query Creates a Relation

• A query pulls information from one or more relations

• A query creates (temporarily) a new relation

• A “virtual” table

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Displaying a Subset of Columns

• To show specific columns, list the column names in the SELECT clause

SELECT First, Last

FROM Tech

• Returns two columns and all rows

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Displaying a Subset of Rows

• To filter rows, add a WHERE clause

SELECT *

FROM Tech

WHERE PayScale = 2

• Returns all columns• Returns only the rows for Techs who have

a PayScale of 2.

Practice

• Display all columns for all jobs• Display the TicketNum, Date, and

Description for all jobs• Display all columns for jobs performed

by TechID 1• Display the TicketNum, Date, and

Description for jobs performed by TechID 1

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Showing Unique Values

• The DISTINCT keyword • Added to the SELECT statement to prevent

rows with duplicate values from displaying

SELECT DISTINCT Location

FROM Job

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Specifying Search Criteria

• The WHERE clause stipulates the matching criteria for the records that are to be displayed

SELECT EmpName

FROM EMPLOYEE

WHERE DeptID = 15

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Match Criteria

• The WHERE clause match criteria may include– Equals “=“– Not Equals “<>”– Greater than “>”– Less than “<“– Greater than or Equal to “>=“– Less than or Equal to “<=“

Strings and Dates

• Strings need to be quoted

SELECT * FROM Job

WHERE Description = “Upgrade Office”

• Dates are represented with the “#” sign

SELECT * FROM Job

WHERE Date > #4/2/2007#

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NULL values

• IS NULL– matches empty values

SELECT * FROM JobWHERE TechID IS NULL

• IS NOT NULL– matches non-empty values

SELECT * FROM JobWHERE TechID IS NOT NULL

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Logical Operators

• Multiple matching criteria may be specified using– AND

• Both conditions must be true

– OR• Either condition may be true

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Operator Examples

SELECT First, Last

FROM Tech

WHERE PayScale > 1 AND PayScale < 3

SELECT Description

FROM Job

WHERE Date > #1/1/2008#

OR TechID = 1

Practice

• Display the dates and descriptions for all jobs before 1/1/2008.

• Display the dates and descriptions for all jobs between 1/1/2007 and 1/1/2008.

• Display the dates and descriptions for all jobs performed by techs 1 and 2.

• Display the dates and descriptions for all jobs performed by techs 1, 2 or 3.

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A List of Values

• The WHERE clause may include the IN keyword• Specifies that a particular column value must be

included in a list of values• Alternative to logical OR

SELECT *

FROM Job

WHERE TechID IN (1, 2, 3);

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The Logical NOT Operator

• Any criteria statement may be preceded by a NOT operator

• All information will be shown except the information matching the specified criteria

SELECT *

FROM Job

WHERE TechID NOT IN (1, 2, 3);

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Finding Data in a Range of Values

• BETWEEN keyword• Allows a user to specify a minimum and

maximum value

SELECT *

FROM Tech

WHERE PayScale BETWEEN 1 AND 3;

• Alternative to:– PayScale >= 1 AND PayScale <= 3

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Wildcard Searches

• SQL LIKE keyword • Allows searches on partial data values• LIKE can be paired with wildcards to find

rows matching a string value– Multiple character wildcard character

• asterisk (*)

– Single character wildcard character • question mark (?)

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Wildcard Search Examples

SELECT First, Last

FROM Tech

WHERE Last LIKE ‘B????'

SELECT *

FROM Job

WHERE Description LIKE ‘*printer*’

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Sorting the Results

• Query results may be sorted using the ORDER BY clause

SELECT *

FROM Job

ORDER BY Description

• Use DESC to sort in descending order

ORDER BY Description DESC

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Built-in SQL 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

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Built-in Function Examples

SELECT COUNT(TechID)

FROM Job

SELECT MIN(PayScale) AS Minimum_Scale,

MAX(PayScale) AS Maximum_Scale,

AVG(PayScale) AS Average_Scale

FROM Tech

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Providing Subtotals

• Subtotals may be calculated by using the GROUP BY clause

• The HAVING clause may be used to restrict which data is displayed

SELECT TechID,

COUNT(*) AS NumberOfJobs

FROM Job

GROUP BY TechID

HAVING COUNT(*) >= 2;

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Retrieving Information from Multiple Tables

• Joins– Combines data across tables

SELECT Date, Description, First, Last

FROM Tech, Job

WHERE Tech. ID = Job.TechID

SELECT Date, Description, First, Last

FROM Tech, Job

WHERE Tech. ID = Job.TechID

AND Last = “Bowers”

Practice

• Display the names of techs whose first names start with A

• Display the names of all techs, sorted by last name

• Count how many jobs have been completed

• Display the names of the techs who completed jobs in 2007.

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