SQL GROUP by usage and GROUP by syntax

SQL and GROUP by are used to group data, and we look at the database table called Employeehours to store the daily time of each company's employees:

Employee	Date	Hours
John Smith	5/6/2004	8
Allan Babel	5/6/2004	8
Tina Crown	5/6/2004	8
John Smith	5/7/2004	9
Allan Babel	5/7/2004	8
Tina Crown	5/7/2004	10
John Smith	5/8/2004	8
Allan Babel	5/8/2004	8
Tina Crown	5/8/2004	9

If the company manager wants to get all of the employees ' working hours superimposed, he needs to execute the following SQL statements:

SELECT SUM (Hours)
From Employeehours

But what if the manager wants to get the sum of his employees for every time?
To do this, he needs to modify his SQL query and the GROUP by statement using sql:

SELECT Employee, SUM (Hours)
From Employeehours
GROUP by Employee

The result of the SQL expression above would be the following:

Employee	Hours
John Smith	25
Allan Babel	24
Tina Crown	27

The SQL's GROUP by clause can be used for other SQL aggregate functions, such as SQL avg:

SELECT Employee, AVG (Hours)
From Employeehours
GROUP by Employee

Employee	Hours
John Smith	8.33
Allan Babel	8
Tina Crown	9

In the date column of our employee table, we can also group

SELECT Date, SUM (Hours)
From Employeehours
GROUP by Date

This is the result of the above SQL expression:

Date	Hours
5/6/2004	24
5/7/2004	27
5/8/2004	25