Dynamically add previous row value to current row value in SQL(Postgres)

Question

I have a table with the following structure:

id	col1	col2
1	1	0
2	1	1
3	1	0
4	1	2
5	1	1
6	1	2
7	1	1

I would like to get the following result:

id	col1	col2	col3
1	1	0	1
2	1	1	1
3	2	0	3
4	0	2	1
5	3	1	3
6	0	2	1
7	0	1	0

where col3= col3 value of previous row + col1 value - col2 value

I added a column of zeroes like this:

id	col1	col2	col3
1	1	0	0
2	1	1	0
3	2	0	0
4	0	2	0
5	3	1	0
6	0	2	0
7	0	1	0

and tried the following query:

select col1, col2, 
lag(col3) over (order by id) + col1 - col2 as col3
from t1;

However I am not able to obtain the desired result. I would like to get some help with this.

Answer 1

You just need analytical function sum as follows:

select col1, col2, 
       Sum(col1 - col2) over (order by id) as col3
  from t1;