Calculate running total in SQL Server 2012

We will use the following Employees table for the examples in this video.


SQL Script to create Employees table
Create Table Employees

(

     Id int primary key,

     Name nvarchar(50),

     Gender nvarchar(10),

     Salary int

)

Go

Insert Into Employees Values (1, 'Mark', 'Male', 5000)

Insert Into Employees Values (2, 'John', 'Male', 4500)

Insert Into Employees Values (3, 'Pam', 'Female', 5500)

Insert Into Employees Values (4, 'Sara', 'Female', 4000)

Insert Into Employees Values (5, 'Todd', 'Male', 3500)

Insert Into Employees Values (6, 'Mary', 'Female', 5000)

Insert Into Employees Values (7, 'Ben', 'Male', 6500)

Insert Into Employees Values (8, 'Jodi', 'Female', 7000)

Insert Into Employees Values (9, 'Tom', 'Male', 5500)

Insert Into Employees Values (10, 'Ron', 'Male', 5000)

Go

SQL Query to compute running total without partitions

SELECT Name, Gender, Salary,

        SUM(Salary) OVER (ORDER BY ID) AS RunningTotal

FROM Employees



SQL Query to compute running total with partitions
SELECT Name, Gender, Salary,

        SUM(Salary) OVER (PARTITION BY Gender ORDER BY ID) AS RunningTotal

FROM Employees



What happens if I use order by on Salary column
If you have duplicate values in the Salary column, all the duplicate values will be added to the running total at once. In the example below notice that we have 5000 repeated 3 times. So 15000 (i.e 5000 + 5000 + 5000) is added to the running total at once. 

SELECT Name, Gender, Salary,

        SUM(Salary) OVER (ORDER BY Salary) AS RunningTotal

FROM Employees



So when computing running total, it is better to use a column that has unique data in the ORDER BY clause.