Aggregate functions in sql

Yet More SQL SELECT
Database Systems Lecture 9
Natasha Alechina

In This Lecture
• Yet more SQL
• ORDER BY
• Aggregate functions
• GROUP BY and HAVING
• UNION etc.
• For more information
• Connoly and Begg Chapter 5
• Ullman and Widom Chapter 6.4

SQL SELECT Overview
SELECT
[DISTINCT | ALL] <column-list>
FROM <table-names>
[WHERE <condition>]
[ORDER BY <column-list>]
[GROUP BY <column-list>]
[HAVING <condition>]
([]- optional, | - or)

ORDER BY
• The ORDER BY clause
sorts the results of a
query
• You can sort in
ascending (default) or
descending order
• Multiple columns can
be given
SELECT <columns>
FROM <tables>
WHERE <condition>
ORDER BY <cols>
[ASCENDING |
DESCENDING|
ASC | DESC ]

ORDER BY Example
Grades
Name Code Mark
John DBS 56
John IAI 72
Mary DBS 60
Mark PR1 43
Mark PR2 35
Jane IAI 54
Name Code Mark
Mark PR2 35
Mark PR1 43
Jane IAI 54
John DBS 56
Mary DBS 60
John IAI 72
SELECT * FROM Grades
ORDER BY Mark

ORDER BY Example
Grades
Name Code Mark
John DBS 56
John IAI 72
Mary DBS 60
Mark PR1 43
Mark PR2 35
Jane IAI 54
Name Code Mark
Mary DBS 60
John DBS 56
John IAI 72
Jane IAI 54
Mark PR1 43
Mark PR2 35
SELECT * FROM Grades
ORDER BY Code ASC,
Mark DESC

Constants and Arithmetic
• As well as column
names, you can
select constants,
compute arithmetic
expressions and
evaluate functions in
a SELECT statement
SELECT Mark/100
FROM Grades
SELECT
Salary + Bonus
FROM Employee
SELECT 1.175*Price
FROM Products

Aggregate Functions
compute summaries
of data in a table
• Most aggregate
functions (all except
COUNT) work on a
single column of
numeric data
• Use an alias to name
the result
• COUNT: The number of
rows
• SUM: The sum of the
entries in a column
• AVG: The average
entry in a column
• MIN, MAX: The
minimum and
maximum entries in a
column

Aggregate Functions
Grades
Name Code Mark
John DBS 56
John IAI 72
Mary DBS 60
Mark PR1 43
Mark PR2 35
Jane IAI 54
SELECT
COUNT(*) AS Count
FROM Grades
SELECT
SUM(Mark) AS Total
FROM Grades
SELECT
MAX(Mark) AS Best
FROM Grades
Count
6
Total
320
Best
72

Aggregate Functions
• You can combine
aggregate functions
using arithmetic
SELECT
MAX(Mark)-MIN(Mark)
AS Range
FROM Grades
Grades
Name Code Mark
John DBS 56
John IAI 72
Mary DBS 60
Mark PR1 43
Mark PR2 35
Jane IAI 54
Range
37
MAX(Mark) = 72
MIN(Mark) = 35

Example
• Find John’s average
mark, weighted by
the credits of each
module
Modules
Code Title Credits
DBS Database Sys. 10
GRP Group Project 20
PRG Programming 10
Grades
Name Code Mark
John DBS 60
Mark GRP 47
Mary PRG 56
SELECT
SUM(Mark*Credits)/SUM(Credits)
FROM Modules, Grades
WHERE Modules.Code=Grades.Code
AND Grades.Name = ‘John’

GROUP BY
• Sometimes we want
to apply aggregate
functions to groups
of rows
• Example, find the
average mark of
each student
• The GROUP BY clause
does this
SELECT <cols1>
FROM <tables>
GROUP BY <cols2>

GROUP BY
SELECT <cols1>
FROM <tables>
GROUP BY <cols2>
• Every entry in
<cols1> must be in
<cols2>, be a
constant, or be an
aggregate function
• You can have WHERE
and ORDER BY
clauses as well as a
GROUP BY clause

GROUP BY
Grades
Name Code Mark
John DBS 56
John IAI 72
Mary DBS 60
Mark PR1 43
Mark PR2 35
Jane IAI 54
SELECT Name,
AVG(Mark) AS Average
FROM Grades
GROUP BY Name
Name Average
John 64
Mary 60
Mark 39
Jane 54

GROUP BY
• Find the total value
of the sales for each
department in each
month
• Can group by Month
then Department or
Department then
Month
• Same results, but in a
different order
Month Department Value
March Fiction 20
March Travel 30
March Technical 40
April Fiction 10
April Fiction 30
April Travel 25
April Fiction 20
May Fiction 20
May Technical 50
Sales

GROUP BY
Month Department Total
April Fiction 60
April Travel 25
March Fiction 20
March Technical 40
March Travel 30
May Fiction 20
May Technical 50
SELECT Month, Department,
SUM(Value) AS Total
FROM Sales
GROUP BY Month, Department
Month Department Total
April Fiction 60
March Fiction 20
May Fiction 20
March Technical 40
May Technical 50
April Travel 25
March Travel 30
SELECT Month, Department,
SUM(Value) AS Total
FROM Sales
GROUP BY Department, Month

HAVING
• HAVING is like a
WHERE clause,
except that it applies
to the results of a
GROUP BY query
• It can be used to
select groups which
satisfy a given
condition
SELECT Name,
FROM Grades
GROUP BY Name
HAVING AVG(Mark) >= 40
Name Average
John 64
Mary 60
Jane 54

WHERE and HAVING
• WHERE refers to the
rows of tables, and
so cannot use
aggregate functions
• HAVING refers to the
groups of rows, and
so cannot use
columns which are
not in the GROUP BY
• Think of a query
being processed as
follows:
• Tables are combined
• WHERE clauses
• GROUP BY and
Aggregates
• Column selection
• HAVING clauses
• ORDER BY

UNION, etc.
• UNION, INTERSECT,
and EXCEPT
• These treat the tables
as sets and are the
usual set operators of
union, intersection,
and difference
• We’ll concentrate on
UNION
• Oracle has MINUS
instead of EXCEPT
• They all combine the
results from two
select statements
• The results of the
two selects must
have the same
columns and data
types

UNION
• Find, in a single
query, the average
mark for each
student, and the
average mark overall
Grades
Name Code Mark
Jane IAI 52
John DBS 56
John IAI 72
Mark PR1 43
Mark PR2 35
Mary DBS 60

UNION
• The average for each
student:
SELECT Name,
FROM Grades
GROUP BY Name
• The average overall
SELECT
‘Total’ AS Name,
FROM Grades
• Note - this has the
same columns as the
average by student

UNION
SELECT Name
FROM Grades
GROUP BY Name
UNION
SELECT
'Total' as Name,
FROM Grades
Name Average
Jane 52
John 64
Mark 39
Mary 60
Total 53

A Final Example
• Examiners’ reports
• We want a list of
students and their
average mark
• For first and second
years the average is
for that year
• For finalists it is 40%
of the second year
plus 60% of the final
year average.
• We want the results
• Sorted by year then
average mark (High
to low) then last
name, first name, and
finally ID
• To take into account
the number of credits
each module is worth
• Produced by a single
query

Tables for the Example
Student
ID First Last Year
Module
Code Title Credits
Grade
ID Code Mark YearTaken

We’ll Need a UNION
• Finalists are treated
differently
• Write one query for
the finalists
• Write a second query
for the first and
second years
• Use a UNION to join
them together
<QUERY FOR FINALISTS>
UNION
<QUERY FOR OTHERS>

We’ll need to Join the Tables
• Both of the
subqueries need
information from all
the tables
• The student ID, name
and year
• The marks for each
module and the year
taken
• The number of credits
for each module
• This is a natural join
operation
• We could use a
NATURAL JOIN
statement, and hope
that our version of
SQL can do it
• Safer to just use a
WHERE clause

The Query So Far
SELECT <some information>
FROM Student, Module, Grade
WHERE Student.ID = Grade.ID
AND Module.Code = Grade.Code
AND <student is in third year>
UNION
SELECT <some information>
AND <student is in first or second year>

Information for Finalists
•We need to retrieve
• Compute average
mark, weighted 40-60
across years 2 and 3
• First year marks need
to be ignored
• The ID, Name, and
Year are needed as
they are used for
ordering
• The average is hard
• We don’t have any
statement to separate
years 2 and 3 easily
• We can exploit the
fact that 40 = 20*2
and 60 = 20*3, so
YearTaken and the
weighting have a
simple relationship

Information for Finalists
SELECT Year, Student.ID, Last, First,
SUM((20*YearTaken/100)*Mark*Credits)/120
AS AverageMark
AND YearTaken IN (2,3)
AND Year = 3
GROUP BY Year, Student.ID, First, Last

Information for Other
Students
• Other students are easier than finalists
• We just need to average their marks where
YearTaken and Year are the same
• As before we need the ID, Name, and Year
for ordering

Information for Other
Students
SUM(Mark*Credits)/120 AS AverageMark
AND YearTaken = Year
AND Year IN (1,2)

The Final Query
SUM((20*YearTaken/100)*Mark*Credits)/120 AS AverageMark
WHERE Student.ID = Grade.ID AND Module.Code = Grade.Code
AND YearTaken IN (2,3) AND Year = 3
UNION
SUM(Mark*Credits)/120 AS AverageMark
WHERE Student.ID = Grade.ID AND Module.Code = Grade.Code
AND YearTaken = Year AND Year IN (1,2)
ORDER BY Year desc, AverageMark desc, First, Last, ID

Yet More SQL SELECT
Next Lecture
• Missing Information
• NULLs and three-valued logic
• NULLs and the relational model
• OUTER JOINs
• Default values
• For more information
• Ullman and Widom 6.1.5, 6.1.6, 6.3.8

Aggregate functions in sql

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