5. Group Functions

REPORTING AGGREGATED DATA, USING GROUP
FUNCTIONS.

OBJECTIVES
At the end of this lesson, you will learn:
 How to identify the available group functions
 How to describe the use of group functions in
select statements
 Grouping data, by using the Group By clause and
 How to include or exclude grouped rows, by
using the having clause.

WHAT ARE GROUP FUNCTIONS?
 Group functions are SQL functions, which operate
on a GROUP of rows, and return a result.
 This group of rows may be an entire column or a
column, split into smaller groups.

 This example uses the GROUP function called COUNT, to
count the number of employees that earn a salary
from the company.
AVG & SUM
 The AVG and Sum keyword is used to find the
average and sum –respectively- of a group of
numbers.

EXERCISE
 Query your database for the average salary of all
the employees in the department with where
department_id=90.

MIN & MAX
 The MAX and the MIN functions, will display the
maximum and minimum –respectively- of all the
values in the specified group, or column.
EXAMPLE

STDDEV & VARIANCE
 STDDEV and VARIANCE are used to find the standard
deviation and the variance, of the numbers in the
specified column or group.
COUNT
 The COUNT function counts the number of rows in
the stated group or column. It has three different
variations but each performs the same function
of counting the number of rows in the group.

COUNT (COLUMN_NAME)
 This SQL statement COUNTs all the rows in the
stated column as returns their total number.
FORMAT:
SELECT COUNT (column_name)
FROM table_name;

 This SQL statement COUNTs all the rows in the
employees table and returns the number of rows.

 The SQL statement counts the number of rows in
the manager _id column.
COUNT DISTINCT (COLUMN_NAME)
 Where the same value occurs many times in a
column, the Oracle server COUNTS them all as one
value when the SELECT COUNT keyword is used.

 The Oracle server would count the different
values in the manager_id column instead of the
number of rows.
 From the result of this query, it is obvious, that all
the 107 employees, share only 18 manager-ids

 The DISTINCT keyword, when used with a group
function, will specify only the different rows
available in that group.
 The opposite of the DISTINCT keyword, is the ALL
keyword, and it operates on all the rows in the
group, including duplicates. Only null values are
exempted.

GROUPING DATA
 Initially we said group functions are functions
that operate on a group of rows.
 We also said these group of rows could be an
entire column.
 When we want to specify the order in which the
output of a GROUP function is processed, we use the
GROUP BY keyword.

 The Oracle server goes to the department_id
column and fetches the distinct departments.
 Afterwards, the Oracle server comes to the salary
column and begins to group the rows in the
salary column by their various departments, and
then finally, for each different department_id, the
Oracle server returns the minimum salary.

 When using the GROUP BY clause, one general rule is
that GROUP BY is always followed by a column
name.
 No column alias can be used with the GROUP BY clause.
EXAMPLE:

NESTED GROUPS
The situation may demand a nested GROUP, also referred
to as a sub GROUP.
EXAMPLE:

 In the event of Nested GROUPS, the inner GROUP function
is SELECT before the outer GROUP function.
INCLUDING & EXCLUDING ROWS
 When restricting rows in a SELECT clause, we use the
WHERE keyword. However, when restricting rows in
a GROUP BY clause, we use the HAVING keyword.
 The GROUP BY clause will only return the rows that
meet the HAVING condition.

 All the rows displayed by the Oracle server, have
a department_id column greater than 30.

5. Group Functions

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5. Group Functions