The Language of SQL- P24 pps

Let’s proceed with an example that introduces theGROUP BYkeyword: SELECT GradeType AS 'Grade Type', AVG Grade AS 'Average Grade' FROM Grades GROUP BY GradeType ORDER BY GradeType... The

The third format of the COUNT function allows you to use the DISTINCT

keyword in addition to a column name Here’s an example:

SELECT

COUNT (DISTINCT FeeType) AS 'Number of Fee Types'

FROM Fees

This statement is counting the number of distinct values for the FeeType

col-umn The result is:

Number of Fee Types

3

This means that there are three different values found in the FeeType column

Grouping Data

The previous examples of aggregation functions are interesting, but of somewhat

limited value The real power of the aggregation functions will become evident

after we introduce the concept of grouping data

The GROUP BY keyword is used to separate data returned from a SELECT

statement into any number of groups For example, when looking at the previous

Grades table, you may be interested in analyzing test scores based on the grade

type In other words, you want to separate the data into two separate groups,

quizzes and homework The value of the GradeType column can be used to

determine which group each row belongs to

Once data has been separated into groups, then aggregation functions can be

utilized so that summary statistics for each of the groups can be calculated and

compared

Let’s proceed with an example that introduces theGROUP BYkeyword:

SELECT

GradeType AS 'Grade Type',

AVG (Grade) AS 'Average Grade'

FROM Grades

GROUP BY GradeType

ORDER BY GradeType

The result is:

Grade Type Average Grade

In this example, theGROUP BYkeyword specifies that groups are to be created based on the value of the GradeType column The two columns in theSELECT

columnlist are GradeType and a calculated field that uses theAVGfunction The

GradeType column was included in the columnlist because when creating a

group, it’s usually a good idea to include the column on which the groups are based The ‘‘Average Grade’’ calculated field aggregates values based on all rows

in each group

Notice that the average homework grade has been computed as 86 Even though there is one row with a NULL value for the Homework type, SQL is smart enough to ignore rows with NULL values when computing an average If you want the NULL value to be counted as a 0, then theISNULLfunction can used to convert the NULL to a 0, as follows:

AVG (ISNULL (Grade, 0)) AS 'Average Grade'

It’s important to note that when using aGROUP BYkeyword, all columns in the

columnlist must either be listed as columns in the GROUP BY clause or else be used in an aggregation function Nothing else would make any sense For example, the followingSELECTwould error:

SELECT

GradeType AS 'Grade Type',

AVG (Grade) AS 'Average Grade',

Student AS 'Student'

FROM Grades

GROUP BY GradeType

ORDER BY GradeType

The problem with this statement is that the Student column is not in theGROUP

BYclause, nor is it aggregated in any way Since everything is being presented in groups, SQL doesn’t know what to do with the Student column

Chapter 10 ■ Summarizing Data

102

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

Unlike Microsoft SQL Server and Oracle, the previous statement will not error in MySQL, but will

produce incorrect results.

Multiple Columns and Sorting

The concept of groups can be extended so the groups are based on more than one

column Let’s go back to the last SELECT and add the Student column to the

GROUP BYclause and also to the columnlist It now looks like:

SELECT

GradeType AS 'Grade Type',

Student AS 'Student',

AVG (Grade) AS 'Average Grade'

FROM Grades

GROUP BY GradeType, Student

ORDER BY GradeType, Student

The resulting data is:

Grade Type Student Average Grade

You now see a breakdown not only of grade types, but also of students The

average grades are computed on each group Note that the Homework row for

Kathy shows a NULL value, since she only has one homework row, and that row

has a value of NULL for the grade

The order in which columns are listed in the GROUP BY clause has no

sig-nificance The results would be the same if the clause were:

GROUP BY Student, GradeType

However, as always, the order that columns are listed in theORDER BYclause is meaningful If you switch theORDER BYclause to:

ORDER BY Student, GradeType

then the results are:

Grade Type Student Average

This still looks a bit strange, since it’s difficult to tell at a glance that the data is really sorted by Student and then by Grade Type As a general rule of thumb, it often helps if columns are listed in the same order in which columns are sorted

A more understandableSELECTstatement would be:

SELECT

Student AS 'Student',

GradeType AS 'Grade Type',

AVG (Grade) AS 'Average Grade'

FROM Grades

GROUP BY GradeType, Student

ORDER BY Student, GradeType

The data now looks like:

Student Grade Type Average Grade

Chapter 10 ■ Summarizing Data

104

This is more comprehensible, since the column order corresponds to the sort

order

There’s sometimes a certain confusion as to the difference between theGROUP

BYandORDER BYclauses Just remember that theGROUP BYmerely creates the

groups You still need to use theORDER BY to present your data in the correct

sequence

Selection Criteria on Aggregates

One more topic needs to be added to our discussion of summarizing data Once

groups are created, selection criteria becomes a bit more complex When

apply-ing any kind of selection criteria to aSELECT with aGROUP BY, one has to ask

whether the selection criteria applies to the individual rows or to the entire

group

In essence, theWHERE clause handles selection criteria for individual rows SQL

provides a keyword named HAVING, which allows for selection criteria at the

group level

Returning to the Grades table, let’s say you want to only look at grades on quizzes

that are 70 or higher The grades you’d like to look at are individual grades, so

you can use theWHEREclause, as normal Such aSELECTmight look like:

SELECT

Student AS 'Student',

GradeType AS 'Grade Type',

Grade AS 'Grade'

FROM Grades

WHERE GradeType ¼ 'Quiz'

AND Grade >= 70

ORDER BY Student, Grade

The resulting data is:

Student GradeType Grade