Sum of a column that is already a sum
Case 1:  Do you want the extra column to have only one entry, at the bottom?

Case 2:  Do you want a column with a running total that increases row by row?

For Case 1, there are two approaches:

1a: Do it in your application code; this is probably cleaner.

1b: You want to do it entirely in SQL:

SELECT ... (what you have, plus an extra column with an empty string) ...
UNION ALL
SELECT (a bunch of blanks, then SUM(to get grand total) ...


For Case 2, you need an @variable that keeps the running total:

SELECT ... SUM(e.salary) AS ..., @run := @run + SUM(e.salary) AS ... ...


To get @run initialized, you could either have a separate SET statement or fold it into this query by using a 'cross join':  JOIN ( SELECT @run := 0 ) AS z (with no ON clause).