i have a table with col1, col2, year. I would like to create a query like:

```
SELECT
SUM(col1-col2)
FROM table1
WHERE year = '2015'

```

and query result set to new column 'col3' for year =2016,next would be(for 2017):

```
SELECT
SUM(col1-col2)
FROM table1
WHERE year = '2015' AND year = '2016'

```

, stopped when year is max.