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SQL Basic GROUP BY example


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.