The SQL SUM function returns the total value within a table or group. In this section we’ll use the HumanResource.Employee table for our examples:

Using the data above, SUM calculates the total SickLeaveHours for all employees:
When used with GROUP BY, SUM totals the values within a group. You can see this in this query showing the maximum SickLeaveHours by JobTitle:
Additional SQL SUM Resources
To learn more, check out these useful resources: