Example
It might be easier if you think of GROUP BY as "for each" for the sake of explanation. The query below:
SELECT EmpID, SUM (MonthlySalary)
FROM Employee
GROUP BY EmpID
is saying:
"Give me the sum of MonthlySalary's for each EmpID"
So if your table looked like this:
+-----+-------------+
|EmpID|MonthlySalary|
+-----+-------------+
|1 |200 |
+-----+-------------+
|2 |300 |
+-----+-------------+
Result:
+-+---+
|1|200|
+-+---+
|2|300|
+-+---+
Sum wouldn't appear to do anything because the sum of one number is that number. On the other hand if it looked like this:
+-----+-------------+
|EmpID|MonthlySalary|
+-----+-------------+
|1 |200 |
+-----+-------------+
|1 |300 |
+-----+-------------+
|2 |300 |
+-----+-------------+
Result:
+-+---+
|1|500|
+-+---+
|2|300|
+-+---+
Then it would because there are two EmpID 1's to sum together.
