How to calculate Difference

I saved Table size in a log table . Then now I have These records :InfoDate ---- SizeMB ---- TableNameD1 ----------- 100 --------T1 D1 ----------- 250 --------T2 D1 ----------- 70 ---------T3 D1 ----------- 300 --------T4 D2 ----------- 110 --------T1 D2 ----------- 270 --------T2 D2 ----------- 100 --------T3 D2 ----------- 301 --------T4 ------------------------------------------------------------------------------------------I Need These Result :InfoDate ---- SizeMB ---- TableNameD2-D1 ------- 10 --------T1 D2-D1 ------- 20 --------T2 D2-D1 ------- 30 --------T3 D2-D1 ------- 1 ---------T4 What Do I Do ?