Information Resource Engineering SQL4. Recap - Ordering Output Usually, the order of rows returned...

Information Resource Engineering

SQL4

Recap - Ordering Output Usually, the order of rows returned in a

query result is undefined.

The ORDER BY clause sets the sequence for outputting selected information.

This can either be: Ascending order ASC (default)

Descending order DESC.

If used the ORDER BY must always be the last clause in the SELECT command.

Aggregate (Group) Functions Aggregate functions perform an operation

on a group of rows and return one result. SUM () gives the total of a group of rows,

(satisfying any conditions) of the given column, where the given column is numeric.

AVG () gives the average of the given column. MAX () gives the largest figure in the given

column. MIN () gives the smallest figure in the given

column. COUNT(*) gives the number of rows satisfying

the conditions.

Aggregate (Group) Functions

This query returns how many books are in the Book table.

SELECT COUNT(*) AS CountFROM Book;

Book

Resulting Output

Count

5

ISBN BookName Price

0747550999 Harry Potter 7.00

0575049802 Witches Abroad 6.00

0509856745 Guards Guards 7.00

0497389956 Bart’s World 10.00

0497563856 The Bone Collector

14.00


We could also use an attribute name in the Count function and it would return the same result.

SELECT COUNT(BookName) AS CountFROM Book;

Book Resulting Output

Count

5

ISBN BookName Price

0747550999 Harry Potter 7.00



0497389956 Bart’s World 10.00


14.00


We could also a different alias and it would return the same result.

SELECT COUNT(BookName) AS NumofRecordsFROM Book;


NumofRecords

5

ISBN BookName Price

0747550999 Harry Potter 7.00



0497389956 Bart’s World 10.00


14.00


This query returns the total price of all the books in the Book table.

SELECT SUM (Price) AS Sum

FROM Book;

Book

Resulting Output

Sum

44.00

ISBN BookName Price

0747550999 Harry Potter 7.00



0497389956 Bart’s World 10.00


14.00


This query returns the average price of a book in the Book table.

SELECT AVG (Price) AS AveragePrice

FROM Book;

Book

Resulting Output

AveragePrice

8.80

ISBN BookName Price

0747550999 Harry Potter 7.00



0497389956 Bart’s World 10.00


14.00


This query returns the Minimum price of a book in the Book table.

SELECT MIN (Price) AS MinimumPrice

FROM Book;

Book

Resulting Output

MinimumPrice

6.00

ISBN BookName Price

0747550999 Harry Potter 7.00



0497389956 Bart’s World 10.00


14.00


This query returns the Maximum price of a book in the Book table.

SELECT MAX (Price) AS MaximumPrice

FROM Book;

Book

Resulting Output

MaximumPrice

14.00

ISBN BookName Price

0747550999 Harry Potter 7.00



0497389956 Bart’s World 10.00


14.00


You can return more than one aggregate function on the same attribute.

SELECT SUM (Price) AS Total, AVG (Price) AS Ave

FROM Book;


Total Ave

44.00 8.80

ISBN BookName Price

0747550999 Harry Potter 7.00



0497389956 Bart’s World 10.00


14.00


You can return more than one aggregate function, and not all the aggregate functions need be on the same attribute.

SELECT SUM (NoInStock) AS TotalStock,

AVG (Price) AS AvePrice

FROM Book;


TotalStock AvePrice

44 8.80

ISBN BookName PriceNoInStock

0747550999

Harry Potter 7.00 10

0575049802

Witches Abroad 6.00 12

0509856745

Guards Guards 7.00 7

0497389956

Bart’s World 10.00 10

0497563856

The Bone Collector

14.00 5


We can use Where statements to limit the rows selected

SELECT SUM (Price) AS Total, AVG (Price) AS Ave

FROM Book

WHERE Type = ‘Hardback’;

Book

Resulting Output

Total Ave

24.00 12.00

ISBN BookName Price Type

0747550999

Harry Potter 7.00 Softback

0575049802

Witches Abroad 6.00 Softback

0509856745

Guards Guards 7.00 Softback

0497389956

Bart’s World 10.00

Hardback

0497563856

The Bone Collector

14.00

Hardback

Aggregate (Group) FunctionsUsing the following table, produce the SQL statement that would output the minimum, maximum and average monthly salary of Programmers.

Employee

EmpNo

NameDeptNo

JobMonthlySalary

E001 Fred D1 Salesperson

1,500.00

E002 BamBam

D1 Analyst 2,300.00

E003 Barney D3 Programmer

1,700.00

E004 Wilma D2 Manager 2,000.00

E006 Pebbles

D3 Programmer

1,200.00

E007 Betty D2 Salesperson

1,400.00

E009 Dino D4 Manager 2,500.00


Trying to return the most expensive book by Type

SELECT Type, MAX (Price) AS Maximum

FROM Book;

Book

ERROR

The command is invalid because Type has a value for each row in the table, while MAX (Price) has a single value for the whole table.

Hardback

Hardback

Softback

Softback

Softback

Type

14.00

The Bone Collector

0497563856

10.00

Bart’s World0497389956 7.00Guards Guards0509856745 6.00Witches Abroad0575049802

7.00Harry Potter0747550999

PriceBookNameISBN

Aggregate (Group) Functions Therefore, if we want to use

aggregate functions on ‘groups’ of rows in the table, we need to use the GROUP BY clause.

This selects the maximum price of each type of book in the table.


FROM Book

GROUP BY Type;


Think of it as the table being sorted by the attribute we are ‘grouping’ by

Then each group is treated as a separate table.

THIS DOES NOT HAPPEN PHYSICALLY!!

Book


0747550999


0575049802


0509856745


0497389956


Hardback

0497563856

The Bone Collector

14.00

Hardback


Returning the most expensive book by Type


FROM Book

GROUP BY Type;

TypeMaximum

Hardback

14.00

Softback 7.00

ResultBook


0747550999


0575049802


0509856745


0497389956


Hardback

0497563856

The Bone Collector

14.00

Hardback


Adding another column, Cat, to the table specifying whether the book was a children’s or adult book:

Book

Hardback

Hardback

Softback

Softback

Softback

Type

14.00

The Bone Collector

0497563856

10.00



PriceBookNameISBN

A

A

A

C

C

Cat

Aggregate (Group) Functions The Group By clause causes the table to be ‘split’

(not physically) into related chunks. I.e related by the attribute(s) specified in the Group By clause.

Book

ISBN BookName Price Type Cat

0747550999

Harry Potter 7.00 Softback C

0575049802

Witches Abroad 6.00 Softback A

0509856745

Guards Guards 7.00 Softback A

0497389956


Hardback

C

0497563856

The Bone Collector

14.00

Hardback

A

Group By Type

Aggregate (Group) Functions The Group By clause causes the table to be ‘split’

(not physically) into related chunks. I.e related by the attribute(s) specified in the Group By clause.

Book

ISBN BookName Price Type Cat

0747550999

Harry Potter 7.00 Softback C

0497389956


Hardback

C

0575049802

Witches Abroad 6.00 Softback A

0509856745

Guards Guards 7.00 Softback A

0497563856

The Bone Collector

14.00

Hardback

A

Group By Cat


Book

Produce the SQL statement to display the maximum book price for each category.

Hardback

Hardback

Softback

Softback

Softback

Type

14.00

The Bone Collector

0497563856

10.00



PriceBookNameISBN

A

A

A

C

C

Cat

Aggregate (Group) Functions Therefore, if we try this SQL

statement:

We will receive an error message. We are trying to perform an aggregate function on an ‘ungrouped’ attribute.

SELECT Cat, MAX (Price) AS Maximum

FROM Book

GROUP BY Type;

Aggregate (Group) Functions However, if we wanted the maximum price of each

category of each type of book, we could use more than one attribute in the Group By clause:

SELECT Type,Cat MAX (Price) AS Maximum

FROM Book

GROUP BY Type,Cat;

Book

A

C

A

A

C

Cat

Hardback

Hardback

Softback

Softback

Softback

Type

14.00

The Bone Collector

0497563856

10.00

Bart’s World0497389956

7.00Guards Guards0509856745

6.00Witches Abroad0575049802


PriceBookNameISBN

Type Cat Maximum

Hardback

A 14.00

Hardback

C 12.00

Softback

A 7.00

Softback

C 7.00

Result

CHardback

12.00

The Amber Spyglass

0729863856 CSoftback 4.00Art Attack0997563856



SELECT Type,Cat,MAX (Price) AS Maximum

FROM Book

GROUP BY Type,Cat;

Book

A

C

A

A

C

Cat

HardbackHardback

Softback

Softback

Softback

Type

14.00

The Bone Collector

0497563856 10.0

0Bart’s World049738995

6




PriceBookNameISBN

Type Cat Maximum

Hardback

A 14.00

Hardback

C 12.00

Softback

A 7.00

Softback

C 7.00

Result

CHardback

12.00

The Amber Spyglass

0729863856

CSoftback 4.00Art Attack0997563856



SELECT Type,Cat,MAX (Price) AS Maximum

FROM Book

GROUP BY Type,Cat;

Book

A

C

A

A

C

Cat

HardbackHardback

Softback

Softback

Softback

Type

14.00

The Bone Collector

0497563856 10.0

0Bart’s World049738995

6




PriceBookNameISBN

12.00CHardback

Hardback

Softback

Type

A

C

Cat

14.00

7.00

Maximum

Result

CHardback

12.00

The Amber Spyglass

0729863856

CSoftback 4.00Art Attack0997563856

Softback

A 7.00

Aggregate (Group) FunctionsUsing the following table, produce the SQL statement that would output the minimum, maximum and average monthly salary of all job types.

Employee

EmpNo

Name JobMonthlySalary

E001 Fred Salesperson

1,500.00

E002 BamBam

Analyst 2,300.00

E003 Barney Programmer

1,700.00

E004 Wilma Manager 2,000.00

E006 Pebbles

Programmer

1,200.00

E007 Betty Salesperson

1,400.00

E009 Dino Manager 2,500.00

HAVING The HAVING clause is designed for use

with the GROUP BY clause. WHERE ‘filters’ individual rows. HAVING ‘filters’ groups.

I.e the HAVING clause allows you to restrict the groups you return.

It is usual, and logical, to include the GROUP BY clause before the HAVING clause.

Groups are formed and group functions calculated before the HAVING clause is applied to a SELECT group for output.


EnrolmentNo

Name CourseCode

S01 Homer C001

S02 Marge C002

S03 Lisa C006

S04 Bart C003

S05 Maggie C002

S06 Mo C001

S07 Apu C003

S08 Skinner C001

S09 Millhouse

C004

S10 Lennie C002SELECT CourseCode, COUNT (EnrolmentNo) AS ‘NoOfStudents’

FROM Student

GROUP BY CourseCode;

ResultCourseCode

NoOfStudents

C001 3

C002 3

C006 1

C003 2

C004 1

Student


EnrolmentNo

Name CourseCode

S01 Homer C001

S02 Marge C002

S03 Lisa C006

S04 Bart C003

S05 Maggie C002

S06 Mo C001

S07 Apu C003

S08 Skinner C001

S09 Millhouse

C004

S10 Lennie C002SELECT CourseCode, COUNT (EnrolmentNo) AS ‘NoOfStudents’

FROM Student

GROUP BY CourseCode

HAVING COUNT (EnrolmentNo) > 1;

ResultCourseCode

NoOfStudents

C001 3

C002 3

C003 2

Student

Returning courses with more than 1 student on them:

Aggregate (Group) FunctionsUsing the following table, produce the SQL statement that would output those departments that have only 1 person employed in that department

Employee

EmpNo NameDeptNo

JobMonthlySalary


1,500.00

E002 BamBam

D1 Analyst 2,300.00


1,700.00


E006 Pebbles D3 Programmer

1,200.00


1,400.00


Aggregate (Group) FunctionsUsing the following table, produce the SQL statement that would output those departments whose monthly wage bill is greater than £3,000.

(Monthly wage bill = sum of all employees monthly salaries in that department)

Employee

EmpNo NameDeptNo

JobMonthlySalary


1,500.00

E002 BamBam

D1 Analyst 2,300.00


1,700.00


E006 Pebbles D3 Programmer

1,200.00


1,400.00


In Conclusion We have covered:

Aggregate Functions (Group)

‘Filtering’ Groups

Information Resource Engineering SQL4. Recap - Ordering Output Usually, the order of rows returned...

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