SUM() OVER() is a window function often used to calculate a running total without grouping rows.

 

SUM() OVER() is a window function often used to calculate a running total without grouping rows.

Sample Employees Table

EmployeeID	EmployeeName	Salary
1	Alice	3000
2	Bob	4000
3	Charlie	2500
4	David	5000
5	Emma	3500

Running Total of Salaries

SELECT
    EmployeeID,
    EmployeeName,
    Salary,
    SUM(Salary) OVER (
        ORDER BY EmployeeID
    ) AS RunningTotal
FROM Employees;

Output

EmployeeID	EmployeeName	Salary	RunningTotal
1	Alice	3000	3000
2	Bob	4000	7000
3	Charlie	2500	9500
4	David	5000	14500
5	Emma	3500	18000

How it works

For each row:

• Alice → 3000

• Alice + Bob → 7000

• Alice + Bob + Charlie → 9500

• Alice + Bob + Charlie + David → 14500

• Alice + Bob + Charlie + David + Emma → 18000

The window function is effectively doing:

SUM(Salary) OVER (
    ORDER BY EmployeeID
    ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
)

which means:

• Start from the first row (UNBOUNDED PRECEDING)

• Keep adding values up to the current row (CURRENT ROW)

Running Total by Department

Suppose the table has a department column:

EmployeeID	EmployeeName	Department	Salary
1	Alice	HR	3000
2	Bob	HR	4000
3	Charlie	IT	2500
4	David	IT	5000
5	Emma	HR	3500

SELECT
    EmployeeID,
    EmployeeName,
    Department,
    Salary,
    SUM(Salary) OVER (
        PARTITION BY Department
        ORDER BY EmployeeID
    ) AS DeptRunningTotal
FROM Employees;

Output

EmployeeID	EmployeeName	Department	Salary	DeptRunningTotal
1	Alice	HR	3000	3000
2	Bob	HR	4000	7000
5	Emma	HR	3500	10500
3	Charlie	IT	2500	2500
4	David	IT	5000	7500

Here, PARTITION BY Department restarts the running total for each department.

Formula:

SUM(column_name) OVER (
    [PARTITION BY partition_column]
    ORDER BY order_column
)

This is one of the most common SQL interview questions involving window functions.