Use the DISTINCT keyword to suppress the counting of any duplicate values within a column.
The example on the slide displays the number of distinct department values in the EMPLOYEES table
All group functions ignore null values in the column. In the slide example, the average is calculated based only on the rows in the table where a valid value is stored in the COMMISSION_PCT column.
The average is calculated as the total commission paid to all employees divided by the number of employees receiving a commission (four).
The example on the slide displays the number of distinct department values in the EMPLOYEES table
All group functions ignore null values in the column. In the slide example, the average is calculated based only on the rows in the table where a valid value is stored in the COMMISSION_PCT column.
The average is calculated as the total commission paid to all employees divided by the number of employees receiving a commission (four).