Lab 3 Aggregating data - Group functions

1. AGGREGATING DATA
USING GROUP FUNCTIONS
Structured Query Language

2. Group Functions
What are group functions ?
Group functions operate on sets of rows to give one result per
group. These sets may be the whole table or the table split into
groups.
“maximum
salary in
the EMP table”

3. Types of Group Functions
• AVG
• COUNT
• MAX
• MIN
• SUM
Function Description
AVG(DISTINCT|ALL|n) Average value of n,
ignoring null values
COUNT({*|DISTINCT|ALL|expr}) Number of rows, where
expr evaluates to
something other than null
(Count all rows using *,
including duplicates and
rows with nulls)
MAX(DISTINCT|ALL|expr) Maximum value of expr,
ignoring null values
MIN(DISTINCT|ALL|expr) Minimum value of expr,
ignoring null values
SUM(DISTINCT|ALL|n) Sum values of n, ignoring
null values

4. Using Group Functions
 Syntax:
SELECT [column, ] group_function(column)
FROM table
[WHERE condition]
[GROUP BY column]
[ORDER BY column];

5. Using AVG and SUM Functions
 We can use AVG, SUM, MIN, and MAX functions against
columns that can store numeric data. The example displays
the average, highest, lowest, and sum of monthly salaries
for all salespeople.

6. Using MIN and MAX Functions
 We can use MIN and MAX for any data type.

7. Using COUNT Function
 COUNT(*) returns the number of rows in a table
 COUNT(expr) returns the number of non-null rows.

8. Using COUNT Function (continued)
 Display the number of departments in the EMP
table.
 Display the number of distinct departments in the
EMP table.

9. Creating Groups of Data
 We have used the group functions on the tables as one large
group of information. At times we need to divide the table
into smaller groups. This can be done by using GROUP BY
clause.
EMP
“average
salary in
EMP table
for each
department”
2916.6667
2175
1566.6667

10. Creating Groups of Data:
GROUP BY Clause
 Divide rows in a table into smaller groups by using th
GROUP BY clause.
SELECT [column, ] group_function(column)
FROM table
[WHERE condition]
[GROUP BY group_by_expression]
[ORDER BY column];
 group_by_expression specifies columns whose values
determine the basis for grouping
rows.

11. Using the GROUP BY Clause
 All columns int the SELECT list that are not in group
functions must by in the GROUP BY clause.
 The GROUP BY column does not have to be in the
SELECT list.

12. Grouping by More Than One Column
EMP
“sum salaries in
the EMP table
for each job,
grouped by
department”
• Display the total salary being paid to each job title,
within each department.

13. Using the GROUP BY Clause
on Multiple Columns
• Display the total salary being paid to each job title,
within each department.

14. Illegal Queries
Using Group Functions
 Any column or expression in the SELECT list that is
not an aggregate function, must be in the GROUP BY
clause.
 Display the number of employees in each
department.

15. Illegal Queries
Using Group Functions
 We cannot use the WHERE clause to restrict groups.
 We use the HAVING clause to restrict groups.
 Display the average salaries of those departments
that have an average salary greater than $2000.

16. Excluding Group Results
 In the same way that we use the WHERE clause to restrict
rows that we select, we use the HAVING clause to restrict
groups.
 To find the maximum salary of each department, but show
only the departments that have maximum salary more than
$2900, we need to do the following:
 Find the maximum salary for each department by grouping by
department number.
 Restrict the groups to those departments with a maximum salary
greater than $2900.

17. Excluding Group Results
EMP
5000
3000
2850
“maximum salary
per department
greater than $2900”

18. Excluding Group Results:
Having Clause
 Use the HAVING clause to restrict groups
 Rows are grouped.
 The group function is applied.
 Groups matching the HAVING clause are displayed.
SELECT column, group_function
FROM table
[WHERE condition]
[GROUP BY group_by_expression]
[HAVING group_condition]
[ORDER BY column];

19. Using the HAVING Clause
 Display department numbers and maximum salary
for those department whose maximum salary is
greater than $2900.

20.  Display the job title and total monthly salary for each
job title with a total payroll exceeding $5000.
Exclude salespeople and sort the list by the total
monthly salary.
Using the HAVING Clause

21. Nesting Group Functions
 Display the maximum average salary.

22. Summary
SELECT column, group_function(column)
FROM table
[WHERE condition]
[GROUP BY group_by_expression]
[HAVING group_condition]
[ORDER BY column];
Order of evaluation of the clauses:
 WHERE clause
 GROUP BY clause
 HAVING clause