The Language of SQL- P23 docx

Here’s an example of the DISTINCT keyword, used with the following SongTitles table: 4 The Rolling Stones Let It Bleed Monkey Man Let’s say you want to see a list of artists in the table

Here’s an example of the DISTINCT keyword, used with the following SongTitles table:

4 The Rolling Stones Let It Bleed Monkey Man

Let’s say you want to see a list of artists in the table This can be accom-plished with:

SELECT

DISTINCT

Artist

FROM SongTitles

ORDER BY Artist

The results are:

Artist

Paul McCartney

The Beatles

The Rolling Stones

The DISTINCT keyword is always placed immediately after the SELECT key-word The DISTINCT specifies that only unique values of the columnlist that

follow are to be brought back In this case, there are only three unique artists, so only three rows are returned

If you want to see unique combinations of both artists and albums, you’d issue: SELECT

DISTINCT

Artist,

Chapter 10 ■ Summarizing Data

96

FROM SongTitles

ORDER BY Artist, Album

and the results would be:

The Rolling Stones Flowers

The Rolling Stones Let It Bleed

Notice that Abbey Road is only listed once, even though there are two songs from

that album in the table This is because the DISTINCT keyword causes only

unique values from the listed columns to be shown

Aggregate Functions

The functions we discussed in Chapter 4 were all scalar functions These

functions were all performed on a single number or value In contrast, aggregate

functions are meant to be used with groups of data The mostly widely used

aggregate functions areCOUNT,SUM,AVG,MIN, andMAX These provide counts,

sums, averages, and minimum and maximum values of groups of data

All of our aggregate function examples will be taken from the following two

tables with data about students, fees, and grades The Fees table contains:

Here’s the Grades table:

Starting with theSUMfunction, let’s say that you want to see the total amount of gym fees paid by all students This can be accomplished with:

SELECT

SUM (Fee) AS 'Total Gym Fees'

FROM Fees

WHERE FeeType ¼ 'Gym'

The resulting data is:

Total Gym Fees

60

As can be seen, theSUM function sums up the total values for the Fee column, subject to selection in the WHERE clause Since the only expression in the

columnlist is an aggregate function, the query returns only one row of data,

giving the aggregate amount

D A T A B A S E D I F F E R E N C E S : M y S Q L

As noted in Chapter 4, MySQL sometimes requires that there be no space between a function name and the left parenthesis This is also true of most aggregate functions For example, the previous statement needs to be written as the following in MySQL:

SELECT

SUM(Fee) AS 'Total Gym Fees'

Chapter 10 ■ Summarizing Data

98

FROM Fees

WHERE FeeType ¼ 'Gym'

TheAVG, MIN, and MAX functions are quite similar Here’s an example of the

AVGfunction In this case, we’re seeking to obtain the average grade of all quizzes

in the Grades table:

SELECT

AVG (Grade) AS 'Average Quiz Score'

FROM Grades

WHERE GradeType ¼ 'Quiz'

The result is:

Average Quiz Score

77

More than one aggregate function can be used in a single SELECT statement

Here’s aSELECTthat illustratesAVG,MIN, andMAX in a single statement:

SELECT

AVG (Grade) AS 'Average Quiz Score',

MIN (Grade) AS 'Minimum Quiz Score',

MAX (Grade) AS 'Maximum Quiz Score'

FROM Grades

WHERE GradeType ¼ 'Quiz'

The result is:

Average Quiz Score Minimum Quiz Score Maximum Quiz Score

The numbers you see have been computed separately The output shows the

average, minimum, and maximum of all quizzes in the Grades table

The COUNT Function

The COUNT function is slightly more complex, in that it can be used in three

different ways

First, the COUNT function can be used to return a count of all selected rows, regardless of the values in any particular column As an example, the following returns a count of all rows with homework grades:

SELECT

COUNT (*) AS 'Count of Homework Rows'

FROM Grades

WHERE GradeType ¼ 'Homework'

The result is:

Count of Homework Rows

3

The asterisk in the parentheses means ‘‘all columns.’’ SQL retrieves all columns

in those rows that are selected, and then it returns a count of the number

of rows

In the second format of theCOUNTfunction, a specific column is specified rather than the asterisk Here’s an example:

SELECT

COUNT (Grade) AS 'Count of Homework Scores'

FROM Grades

WHERE GradeType ¼ 'Homework'

The result is:

Count of Homework Scores

2

Notice the subtle difference between the previous twoSELECTstatements In the first, you’re merely counting rows where the GradeType equals ‘‘Homework.’’ There are three of these rows In the second, you’re counting occurrences of the Grade column where the GradeType column has a value of ‘‘Homework.’’ In this case, one of the three rows has a value of NULL in the Grade column, so it isn’t counted As you remember, NULL means that the data doesn’t exist

Chapter 10 ■ Summarizing Data

100