SELECT
    wlc.WorkloadCategory
    ,wl.WorkloadMinutes
FROM
    swam.Assumptions assum
CROSS JOIN
    swam.WorkloadCategories  wlc
INNER JOIN
    swam.Workloads wl
        ON wlc.WorkloadCategoryID = wl.WorkloadCategoryID
WHERE
    wlc.WorkloadCategory = 'DI'
GROUP BY
    wlc.WorkloadCategory
    ,wl.WorkloadMinutes WITH ROLLUP

WorkloadCategory    |   WorkloadMinutes
--------------------|------------------
DI                  |   31
DI                  |   55
DI                  |   77
DI                  |   79
DI                  |   98
DI                  |   117
DI                  |   NULL
NULL                |   NULL

WorkloadCategory    |   WorkloadMinutes
--------------------|------------------
DI                  |   31
DI                  |   55
DI                  |   77
DI                  |   79
DI                  |   98
DI                  |   117
DI                  |   457
NULL                |   457

SELECT
    wlc.WorkloadCategory
    ,SUM(wl.WorkloadMinutes) as WorkloadMinutes
FROM
    swam.Assumptions assum
CROSS JOIN
    swam.WorkloadCategories  wlc
INNER JOIN
    swam.Workloads wl
        ON wlc.WorkloadCategoryID = wl.WorkloadCategoryID
WHERE
    wlc.WorkloadCategory = 'DI'
GROUP BY
    wlc.WorkloadCategory
    ,wl.WorkloadMinutes WITH ROLLUP

WorkloadCategory    |   WorkloadMinutes
--------------------|------------------
DI                  |   31
DI                  |   55
DI                  |   77
DI                  |   79
DI                  |   196
DI                  |   117
DI                  |   555
NULL                |   555