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A Guide to SQL, Sixth Edition

Chapter 3Single-Table Queries

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Objectives

Retrieve data from a database using SQL commands

Use compound conditions

Use computed columns

Use the SQL LIKE operator

Use the SQL IN operator

Sort data using the ORDER BY clause

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ObjectivesSort data using multiple keys and in ascending and descending orderUse SQL aggregate functionsUse subqueriesGroup data using the GROUP BY clauseSelect individual groups of data using the HAVING clauseRetrieve columns with null values

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Simple Queries

Query - a question represented in a way that the DBMS can understand

Basic formatSELECT-FROM

OptionalSELECT-FROM-WHERE

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SELECT Command

SELECT clause Followed by columns to be included in the query

FROM clause Followed by name of the table that contains the

data to query

WHERE clause (optional) Followed by conditions that apply to the data to be

retrieved

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Retrieving Certain Columns and All Rows

List columns to include in the SELECT clause and the table name in the FROM clause

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Retrieving All Columns and All Rows

An asterisk (*) indicates that you want to include all columns

The result shows all columns in the order in which they were described to the system when the table was created

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Using a WHERE Clause

The WHERE clause retrieves rows that satisfy some condition

A simple condition has the form:Column name, comparison operator, either

another column name or a value

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Comparison Operators

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Note

Generally SQL is not case-sensitive

Exception:Values within quotation marks

Use the correct case for these values

Example:WHERE LAST ‘adams’ will not select any

rows if the stored value is “Adams”

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Compound Conditions

Compound conditionsFormed by connecting two or more simple

conditionsUses of AND, OR, and NOT operators

AND: all conditions must be trueOR: any one of the conditions is trueNOT: reverses the truth of the original condition

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Use of BETWEEN

BETWEEN operatorNot an essential featureCan arrive at same answer without it using

ANDDoes make certain SELECT commands

simpler

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Computed Columns

Computed columnsDo not exist in the databaseCan be computed using data in existing

columnsUse arithmetic operators

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Arithmetic Operators

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Use of LIKE

LIKE operator is used when exact matches will not work

Use LIKE with a wildcard symbol

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Wildcard Symbols

Percent symbol (%) represents any collection of characters ‘%Pine%’

Underscore (_)Represents any individual character ‘T_m’

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Use of IN

The IN clause provides a concise way of phrasing certain conditions

The IN clause consists of the IN operator followed by a collection of values

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Sorting

Generally, the order of rows is immaterial to the DBMS

There is no defined order in which results are displayed

In SQL, the results order can be specified by using the ORDER BY clause

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Use of ORDER BY

Use the ORDER BY command to list data in a specific orderThe ORDER BY clause is followed by the sort key The column on which data is to be sorted is called a sort key or simply keyIf a sort order is not specified, the default is ascending

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Additional Sorting Options

When sorting more than one column: The more important column is called the major key

(or primary sort key) The less important column is called the minor key

(or secondary sort key)

List keys in the order of importance in the ORDER BY clause

Sort in a descending order by using the DESC operator

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Using Functions

SQL has special functions, called aggregate functions, to calculate:SumsAveragesCountsMaximum valuesMinimum values

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SQL Functions

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Use of COUNT Function

Count function counts the number of rows in a table

The specific row to be counted is not important because each count should provide the same answer

Most implementations of SQL use the asterisk (*) to represent any column

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Use of the SUM Function

The SUM function is used to calculate totals

The column to be totaled must be specified

The column to be totaled must be numeric

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Using AVG, MAX, and MIN

AVG, MAX and MIN functions are similar to the SUM function

SUM, AVG, MAX, and MIN functions ignore (eliminate) null valuesNull values can cause strange results

when calculated

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Use of DISTINCT

DISTINCT operator is not a function

Useful when used in conjunction with COUNT function

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Nesting Queries

Sometimes obtaining the results you need is a two-step process (or more)

Problem:List the number of each part in class APList the order numbers that contain an

order line for a part in class AP

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Subqueries

It is possible to place one query inside another

An inner query is called a subquery and it is evaluated first

An outer query can use the results of the subquery to find its results

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Grouping

Grouping creates groups of rows that share some common characteristics

Calculations are performed for the entire group

Use the GROUP BY command

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Using GROUP BY

GROUP BY command allows data to be grouped in a particular order

Statistics are calculated on the groups

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Using a HAVING Clause

The HAVING clause is used to restrict the groups that will be included

HAVING vs. WHEREWHERE clause limits rowsHAVING clause limits groups

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NullsSometimes a condition involves a column that can be null 

Problem: List the number and name of each customer with a

null (unknown) street value

The correct format of the condition to use for this problem is STREET IS NULL

To select a customer whose street is not null, the condition STREET IS NOT NULL is used

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Summary

The basic form of the SQL SELECT command is SELECT-FROM-WHERE

You can form compound conditions by combining simple conditions

Use the BETWEEN operator to indicate a range of values in a condition

Use the LIKE operator to check for a value in a character column that is similar to a particular string of characters

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SummaryUse the IN operator to determine whether a column contains one of a particular set of values

Use an ORDER BY clause to sort data

SQL processes the aggregate functions COUNT, SUM, AVG, MAX, and MIN

When one SQL query is placed inside another, it is called a subquery

Use a GROUP BY clause to group data

Use a HAVING clause to restrict the output to certain groups

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SQL Project Three Completed

Good Luck

H. Zamanzadeh