Less04Reporting Aggregated Data

After completing this lesson, you should be able to do the following:

Identify the available group functions

Describe the use of group functions

Group data by using the GROUP BY clause

Include or exclude grouped rows by using the HAVING clause

You can use AVG and SUM for numeric data.

SELECT AVG(salary), MAX(salary),

       MIN(salary), SUM(salary)

FROM   employees

WHERE  job_id LIKE '%REP%';

You can use MIN and MAX for numeric, character, and date data types.

SELECT MIN(hire_date), MAX(hire_date)

FROM    employees;

COUNT(*) returns the number of rows in a table:

SELECT COUNT(*)

FROM   employees

WHERE  department_id = 50;

COUNT(expr) returns the number of rows with non-null values for the expr:

SELECT COUNT(commission_pct)

FROM   employees

WHERE  department_id = 80;

COUNT(DISTINCT expr) returns the number of distinct non-null values of the expr.

To display the number of distinct department values in the EMPLOYEES table:

SELECT COUNT(DISTINCT department_id)

FROM   employees;

Group functions ignore null values in the column:

SELECT AVG(commission_pct)

FROM   employees;

SELECT AVG(NVL(commission_pct, 0))

FROM   employees;

All columns in the SELECT list that are not in group functions must be in the GROUP BY clause.

SELECT   department_id, AVG(salary)

FROM     employees

GROUP BY department_id ;

The GROUP BY column does not have to be in the SELECT list.

SELECT   AVG(salary)

FROM     employees

GROUP BY department_id ;

SELECT   department_id dept_id, job_id, SUM(salary)

FROM     employees

GROUP BY department_id, job_id ;

Any column or expression in the SELECT list that is not an aggregate function must be in the GROUP BY clause:

SELECT department_id, COUNT(last_name)

FROM   employees;

You cannot use the WHERE clause to restrict groups.

You use the HAVING clause to restrict groups.

You cannot use group functions in the WHERE clause.

SELECT   department_id, AVG(salary)

FROM     employees

WHERE    AVG(salary) > 8000

GROUP BY department_id;

When you use the HAVING clause, the Oracle server restricts groups as follows:

1.  Rows are grouped.

2.  The group function is applied.

3.  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];

SELECT    column, group_function

FROM      table

[WHERE    condition]

[GROUP BY group_by_expression]

[HAVING   group_condition]

SELECT   department_id, MAX(salary)

FROM     employees

GROUP BY department_id

HAVING   MAX(salary)>10000 ;

SELECT   job_id, SUM(salary) PAYROLL

FROM     employees

WHERE    job_id NOT LIKE '%REP%'

GROUP BY job_id

HAVING   SUM(salary) > 13000

ORDER BY SUM(salary);

Display the maximum average salary: 

SELECT   MAX(AVG(salary))

FROM     employees

GROUP BY department_id;

In this lesson, you should have learned how to:

Use the group functions COUNT, MAX, MIN, and AVG

Write queries that use the GROUP BY clause

Write queries that use the HAVING clause 

This practice covers the following topics:

Writing queries that use the group functions

Grouping by rows to achieve more than one result

Restricting groups by using the HAVING clause