[sql-server] How to find difference between two columns data?

I have a temp table with two columns of integer data i want to find the difference between two columns in 3rd column.

#TEMP1

Present   previous
59           88
75           75
45           45
77           88
09           08

#temp2

Difference
 29
 0
 0
 11 
 -1

Is this possible ??

IF the table is alias t

SELECT t.Present , t.previous, t.previous- t.Present AS Difference
FROM   temp1 as t

select previous, Present, previous-Present as Difference from tablename

or

select previous, Present, previous-Present as Difference from #TEMP1

Yes, you can select the data, calculate the difference, and insert all values in the other table:

insert into #temp2 (Difference)
select previous - Present
from #TEMP1

There are many ways of doing this (and I encourage you to look them up as they will be more efficient generally) but the simplest way of doing this is to use a non-set operation to define the value of the third column:

SELECT
    t1.previous
    ,t1.present
    ,(t1.present - t1.previous) as difference
FROM #TEMP1 t1

Note, this style of selection is considered bad practice because it requires the query plan to reselect the value of the first two columns to logically determine the third (a violation of set theory that SQL is based on). Though it is more complicated, if you plan on using this to evaluate more than the values you listed in your example, I would investigate using an APPLY clause. http://technet.microsoft.com/en-us/library/ms175156(v=sql.105).aspx