MySQL | Sum Column If Different ID - mysql

Question

I'm trying to Sum column only if id is different.
I tried like this, but it is not working:
SUM(CASE WHEN
DISTINCT empLeaves.ApprovedLeaveID
THEN empLeaves.TotalLeavesTaken END) AS Total
What Would be the proper way to SUM it?
Here is the total query, I am working on:
SELECT
E.employee_id AS EmployeeID,
E.employee_code AS EmployeeCode,
E.full_name AS EmployeeName,
empLeaves.LeavesFromDate,
empLeaves.LeavesToDate,
-- COUNT(empLeaves.ApprovedLeaveID) AS TotalApprovedApplications,
ATT.`in_date` AS InDate,
ATT.`out_date` AS OutDate,
SUM(CASE WHEN DISTINCT empLeaves.ApprovedLeaveID THEN empLeaves.TotalLeavesTaken END) AS Total
FROM employee E
LEFT JOIN `attendence` ATT ON ATT.`employee_id` = E.`employee_id`
LEFT JOIN (
SELECT
LA.`leave_approval_id` AS ApprovedLeaveID,
E.`employee_id` AS EmployeeID,
LA.`approved_from` AS LeavesFromDate,
LA.`approved_to` AS LeavesToDate,
DATEDIFF(LA.approved_to,approved_from) AS TotalLeavesTaken
FROM leave_application APP
INNER JOIN leave_approval LA
ON LA.`leave_application_id` = APP.`application_id` AND LA.`trashed` = 0 AND LA.`status` = 2
INNER JOIN employee E
ON E.`employee_id` = APP.`employee_id` AND E.`trashed` = 0 AND E.`enrolled` = 1
INNER JOIN ml_leave_type MLLT
ON MLLT.`ml_leave_type_id` = APP.`ml_leave_type_id` AND MLLT.`trashed` = 0
) empLeaves ON E.`employee_id` = empLeaves.EmployeeID
WHERE
E.`enrolled` = 1 AND E.`trashed` = 0
GROUP BY
E.`employee_id`,empLeaves.EmployeeID

Answer 1

When which ID is different from what?
In general, when you have a value in a field and you want to add them together except for those in a particular set of rows, you simply set the unwanted values to 0:
sum( case otherfield when somevalue then row.field else 0 end ) as SumOfValues
So when otherfield shows that this is the row you want included in the total, you expose the field you want to add (row.field in this example). Otherwise, you expose a zero. Of course, if the value you're testing is to find the rows you don't want in the total, just put the zero after the then and the field after the else.