Page 107 - SQL

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.

ROLAP aggregation (Data Mining)

Description

The SQL standard provides two additional aggregate operators. These use the polymorphic value

https://riptutorial.com/ 89