Update table Data without using while loop in sql

Question

in sql i have table data like below

id      type     amount       
1      type1       2000    
2      type1       1000     
3      type2        500    
4      type3       3000    
5      type1       2000   
6      type2        500        
7      type3       5000    
8      type1       1000    

and i want to get datas in select statement like below

id      type     amount      current   
1      type1       2000         2000                
2      type1       1000         1000                 
3      type2        500          500                 
4      type3       3000         3000                 
5      type1       2000         3000                  
6      type2       -500            0                 
7      type3       5000         2000
8      type1      -1000         4000 

and so on that means each type must have its current total amount based on amount type and its need to be dont have while loop because it takes long time to execute

for eg:

in type 1

ID      Amount      current 
1      2000-add       2000                   
2      1000-sub       1000                  
3      2000-add       3000                   
4      1000-add       4000                   

how to do it

Answer 1

I think this query will work:

select id,type,amount,(select sum(amount) from mytable t1 where t1.type=t2.type and t1.id<=t2.id) currenttotal from mytable t2

Answer 2

A self join is enough:

 select
    t1.id, t1.type, t1.amount, sum(t2.amount) as currenttotal
 from
  t t1 inner join t t2
 on t1.id >= t2.id and t1.type = t2.type
 group by
    t1.id, t1.type, t1.amount
 order by t1.id

Test it at sql fiddle:

| ID |  TYPE | AMOUNT | CURRENTTOTAL |
--------------------------------------
|  1 | type1 |   2000 |         2000 |
|  2 | type1 |   1000 |         3000 |
|  3 | type2 |    500 |          500 |
|  4 | type3 |   3000 |         3000 |
|  5 | type1 |   2000 |         5000 |
|  6 | type2 |    500 |         1000 |
|  7 | type3 |   5000 |         8000 |
|  8 | type1 |  -1000 |         4000 |

Explanation

You can not use windowed functions because you don't aggregate for all rows of a same value but for previous rows of same value. Then, you need a non equi join to same table, you get in join all previous rows ( t1.id >= t2.id ) of same value ( t1.type = t2.type )