I want to compute result from this table. I want quantity 1 - quantity2 as another column in the table shown below. this table has more such records

I am trying to query but not been able to get result.

select  * from v order by is_active desc, transaction_id desc


PK_GUEST_ITEM_ID FK_GUEST_ID          QUANTITY               TRANSACTION_ID       IS_ACTIVE
---------------- -------------------- ---------------------- -------------------- -----------
12963            559                  82000                  795                  1
12988            559                  79000                  794                  0
12987            559                  76000                  793                  0
12986            559                  73000                  792                  0
12985            559                  70000                  791                  0
12984            559                  67000                  790                  0
12983            559                  64000                  789                  0
12982            559                  61000                  788                  0
12981            559                  58000                  787                  0
12980            559                  55000                  786                  0
12979            559                  52000                  785                  0
12978            559                  49000                  784                  0
12977            559                  46000                  783                  0
12976            559                  43000                  782                  0

I want another column that will contain the subtraction of two quantities .

DESIRED RESULT SHOULD BE SOMETHING LIKE THIS

PK_GUEST_ITEM_ID FK_GUEST_ID          QUANTITY     Result          TRANSACTION_ID       IS_ACTIVE
---------------- -------------------- ---------------------- -------------------- -----------
12963            559                  82000       3000           795                  1
12988            559                  79000       3000           794                  0
12987            559                  76000       3000           793                  0
12986            559                  73000       3000           792                  0
12985            559                  70000       3000           791                  0
12984            559                  67000       3000           790                  0
12983            559                  64000       3000           789                  0
12982            559                  61000       3000           788                  0
12981            559                  58000       3000           787                  0
12980            559                  55000       3000           786                  0
12979            559                  52000       3000           785                  0
12978            559                  49000       3000           784                  0
12977            559                  46000       3000           783                  0
12976            559                  43000       NULL           782                  0