1. CS122 Using Relational Databases and SQL
2/23/2019
CS122 Using Relational Databases and SQL
3. Aggregate calculations
Daniel Firpo
Slides prepared by Randy Moss
Department of Computer Science
California State University, Los Angeles

2. Outline Aggregate calculations for a table
2/23/2019
Outline
Aggregate calculations for a table
Aggregate calculations for groups
Function followed by field or expression in parentheses
Can use AS and an alias column name
Can use Aggregate as part of expression
3. Aggregate Calculations CS1222_W2018

3. What is an Aggregate? Summarizes values from entire groups of rows
2/23/2019
What is an Aggregate?
Summarizes values from entire groups of rows
SUM: To calculate totals
AVG: To calculate averages
COUNT: To count the number of records
MIN: To report the minimum value
MAX: To report the maximum value
3. Aggregate Calculations CS1222_W2018

4. Sum Add up a column Only for numeric attributes
2/23/2019
Sum
Add up a column
Only for numeric attributes
Any null values will be ignored.
E.X. Report the total time in seconds of all tracks
SELECT Sum (lengthseconds)
FROM Tracks
3. Aggregate Calculations CS1222_W2018

5. 2/23/2019
Avg
AVG ignores nulls
Report the average length in minutes of all tracks
SELECT Avg(lengthseconds)/60
FROM Tracks
3. Aggregate Calculations CS1222_W2018

6. Avg 30! SELECT Avg(age) FROM Employees The result is 20 or 30? 01 25
2/23/2019
Avg
Employees
eid
age 01 25 02
NULL 03 35
SELECT Avg(age)
FROM Employees
The result is 20 or 30?
30!
3. Aggregate Calculations CS1222_W2018

7. Count Take as an argument either a field name or an asterisk(*)
2/23/2019
Count
Take as an argument either a field name or an asterisk(*)
E.X. Report the number of members in the members table
SELECT Count(*) AS NumMembers
FROM Members
3. Aggregate Calculations CS1222_W2018

8. Count Difference between Count(*) and Count (field) SELECT Count(age)
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Count
Difference between Count(*) and Count (field)
SELECT Count(age)
FROM Employees
Employees 2!
eid
age 01 25 02
NULL 03 35
SELECT Count(*)
FROM Employees 3!
3. Aggregate Calculations CS1222_W2018

9. Min, Max Report the minimum and maximum values.
2/23/2019
Min, Max
Report the minimum and maximum values.
Can be used with numeric datatypes, dates and text
E.X. Report the shortest and the longest track lengths in seconds
SELECT Min(lengthseconds) as Shortest,
Max(lengthseconds) as Longest
FROM Tracks
3. Aggregate Calculations CS1222_W2018

10. Aggregate functions summary
2/23/2019
Aggregate functions summary
Can be used as part of an expression
Use WHERE to limit the number of rows being aggregated
Multiple aggregate functions can be used together
Use AS to give an aggregate function an alias
3. Aggregate Calculations CS1222_W2018

11. Discussions SELECT avg(lengthseconds) FROM Tracks
2/23/2019
Discussions
SELECT avg(lengthseconds)
FROM Tracks
Returns a SINGLE VALUE!
SELECT lengthseconds
Returns a TABLE!
3. Aggregate Calculations CS1222_W2018

12. Discussions SELECT tracktitle, avg(lengthseconds) FROM Tracks
2/23/2019
Discussions
SELECT tracktitle, avg(lengthseconds)
FROM Tracks
Does this query make any sense?
No, we need group by!
3. Aggregate Calculations CS1222_W2018

13. Outline Aggregate calculations for a table
2/23/2019
Outline
Aggregate calculations for a table
Aggregate calculations for groups
3. Aggregate Calculations CS1222_W2018

14. 2/23/2019
What are groups?
Member id
Gender
Region
Country 11 M TX
USA 12 ON
CANADA 13 14 F VA 15 16
A group contains a collection of rows that have the same attribute(s) values.
3. Aggregate Calculations CS1222_W2018

15. What are groups? (cont.) Example groups All male members
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What are groups? (cont.)
Example groups
All male members
Members from USA
Members from Virginia
female members from Ontario
3. Aggregate Calculations CS1222_W2018

16. 2/23/2019
GROUP BY
If SELECT has only aggregates, will report just one row for the entire set of records
If non-aggregate fields are included in SELECT clause, query will report one row for each combination of non-aggregate field values with aggregates calculated for each
Must include a GROUP BY clause listing the non-aggregate fields
The more fields in the GROUP BY clause, the more rows will be reported
Proper order of clauses: SELECT column, Aggregate(column) As column_name FROM tablename WHERE condition GROUP BY column ORDER BY column
3. Aggregate Calculations CS1222_W2018

17. Aggregate a single group
2/23/2019
Aggregate a single group
Format:
Select count|max|min|avg|sum
From table_name
Where conditions
The conditions to form a group
3. Aggregate Calculations CS1222_W2018

18. Aggregate a single group example1:
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Aggregate a single group example1:
Report the number of male members
Select count(*)
From Members
Where gender = ‘m’
3. Aggregate Calculations CS1222_W2018

19. Aggregate a single group example:
2/23/2019
Aggregate a single group example:
Report the number of female members from Ontario
Select count(*)
From Members
Where gender = ‘f’ and region = ‘on’
3. Aggregate Calculations CS1222_W2018

20. Aggregate multiple groups
2/23/2019
Aggregate multiple groups
How to answer the following queries using a single query?
Report the number of male AND female members?
For each distinct combination of gender and region, report the number of members
Use group by!!
3. Aggregate Calculations CS1222_W2018

21. 2/23/2019
GROUP BY
When you include non-aggregate fields with aggregates, you must include a GROUP BY clause listing the non-aggregate fields. One row will be reported for each combination of the non-aggregate fields.
SELECT column, Aggregate (column)
FROM Tablename
WHERE condition
GROUP BY column
ORDER BY column
3. Aggregate Calculations CS1222_W2018

22. What is a group? Report the number of male AND female members?
2/23/2019
What is a group?
Report the number of male AND female members?
SELECT gender, count(*)
FROM Members
GROUP BY gender
Member id
Gender
Region
Country 11 M TX
USA 12 ON
CANADA 13 14 F VA 15 16
Gender Count M F
3. Aggregate Calculations CS1222_W2018

23. GROUP BY Example
2/23/2019
Select Count(*) As Num From Members Where Is Not Null
Num 12
Select Gender, Count(*) As Num From Members Where Is Not Null Group By Gender
Gender Num F M
3. Aggregate Calculations CS1222_W2018

24. 2/23/2019
What is a group?
For each distinct combination of gender and region, report the number of members
SELECT gender, region, count(*)
FROM Members
GROUP BY gender, region
Member id
Gender
Region
Country 11 M TX
USA 12 ON
CANADA 13 14 F VA 15 16
Gender region Count
M TX
M ON
M VA
F VA
F ON
3. Aggregate Calculations CS1222_W2018

25. GROUP BY Example (cont)
2/23/2019
Select Region, Gender, Count(*) As Num From Members Where Is Not Null Group By Region, Gender
Region Gender Num
IN F
VT F
CA M
GA M
IN M
NC M
NY M
OH M
ONT M
TX M
VA M
3. Aggregate Calculations CS1222_W2018

26. GROUP BY Example Report the total time for EACH TITLE ID
2/23/2019
GROUP BY Example
Report the total time for EACH TITLE ID
SELECT titleId, sum(lengthseconds)
FROM Tracks
GROUP BY titleId
3. Aggregate Calculations CS1222_W2018

27. GROUP BY Example (cont.)
2/23/2019
GROUP BY Example (cont.)
Report the shortest and longest track lengths for EACH TITLE ID
SELECT titleId, Min(lengthseconds), Max(lengthseconds)
FROM Tracks
GROUP BY titleId
3. Aggregate Calculations CS1222_W2018

28. GROUP BY Example (cont)
2/23/2019
GROUP BY Example (cont)
Report the number of members for each combination of country and gender
SELECT country, gender, count(*)
FROM Members
GROUP BY country, gender
3. Aggregate Calculations CS1222_W2018

29. 2/23/2019
GROUP BY summary
Aggregate on distinct value(s) of the group-by attribute(s)
Group-by attribute(s) must be the same as the non-aggregate attributes in the SELECT clause
The more fields in the GROUP BY clause, the more rows will be reported
3. Aggregate Calculations CS1222_W2018

30. 2/23/2019
HAVING
HAVING restricts results based on aggregated values (eliminate groups)
HAVING always uses an aggregate function as its test
Proper order of clauses:
SELECT column, Aggregate(column)
FROM tablename WHERE condition GROUP BY column HAVING condition ORDER BY column
3. Aggregate Calculations CS1222_W2018

31. 2/23/2019
HAVING vs. WHERE
WHERE restricts results prior to aggregate calculations based on individual row values
HAVING restricts results based on aggregated values
HAVING always uses an aggregate function as its test
Proper order of clauses: SELECT column, Aggregate(column) As column_name FROM tablename WHERE condition GROUP BY column HAVING condition ORDER BY column
3. Aggregate Calculations CS1222_W2018

32. Base selection with aggregate
2/23/2019
HAVING Example
For each titleID, report the average length of all tracks.
Select TitleID, Avg(lengthseconds) As AvgLength From Tracks Group by TitleID
TitleID AvgLength
Base selection with aggregate
3. Aggregate Calculations CS1222_W2018

33. 2/23/2019
HAVING Example (cont)
Select TitleID, Avg(lengthseconds) As AvgLength From Tracks Group by TitleID Having Avg(lengthseconds)>240
TitleID AvgLength
HAVING eliminates aggregated averages of 240 seconds or less, so calculations match original but with fewer resulting rows
3. Aggregate Calculations CS1222_W2018

34. 2/23/2019
HAVING Example (cont)
Select TitleID, Avg(lengthseconds) As AvgLength From Tracks Where lengthseconds>240 Group by TitleID
TitleID AvgLength
WHERE eliminates detail records with length of 240 seconds or less, so reported averages are higher and no groups are eliminated.
3. Aggregate Calculations CS1222_W2018

35. 2/23/2019
HAVING Example (cont)
Select TitleID, Avg(lengthseconds) As AvgLength From Tracks Where lengthseconds>400 Group by TitleID
TitleID AvgLength
WHERE eliminates detail records with length of 400 seconds or less, so reported averages are higher.
3. Aggregate Calculations CS1222_W2018

36. Summary Aggregate calculations for a table
2/23/2019
Summary
Aggregate calculations for a table
Sum, max, min, avg, count
Aggregate calculations for groups
Group by
having
3. Aggregate Calculations CS1222_W2018