Please give me the query for bellow required output.

Query to make a separate column to differentiate over by over difference?

create table test1(overs int,runs int)
insert into test1 values(1,10)
insert into test1 values(2,17)
insert into test1 values(3,20)
insert into test1 values (4,32)
insert into test1 values(5,38)

O/P
===
 Overs  runs  difference
  1     10     NULL
  2     17     7
  3     20     3 
  4     32     12
  5     38     6

What I have tried:

please any one give me the Query to achieve this above Requirement.