Conditional Logic within SUM - sql

Question

I'm currently combining two tables through a UNION ALL query and performing SUM and GROUP BY operations on the result. Everything is working as expected, but I have a unique requirement which I can't seem to figure out how to implement.
My aim is to write SQL that says "when DEV_AGE column is >= 12 set the REVENUE value to what it would be if this column was 12". I provide the code below as I know this description can be a bit confusing:
REVENUE table:
ACC_YR DEV_AGE STATE REVENUE LOSS
2012 3 MA 4000 0
2012 6 MA 8000 0
2012 9 MA 12000 0
2012 12 MA 16000 0
LOSS table:
ACC_YR DEV_AGE STATE REVENUE LOSS
2012 3 MA 0 2000
2012 6 MA 0 7000
2012 9 MA 0 9000
2012 12 MA 0 10000
2012 15 MA 0 14000
2012 18 MA 0 14000
2012 21 MA 0 14000
2012 24 MA 0 15000
2012 27 MA 0 17000
Table after UNION ALL, GROUP BY, SUM:
ACC_YR DEV_AGE STATE REVENUE LOSS
2012 3 MA 4000 2000
2012 6 MA 8000 7000
2012 9 MA 12000 9000
2012 12 MA 16000 10000
2012 15 MA 0 14000
2012 18 MA 0 14000
2012 21 MA 0 14000
2012 24 MA 0 15000
2012 27 MA 0 17000
What I WANT to accomplish:
ACC_YR DEV_AGE STATE REVENUE LOSS
2012 3 MA 4000 2000
2012 6 MA 8000 7000
2012 9 MA 12000 9000
2012 12 MA 16000 10000
2012 15 MA 16000 14000
2012 18 MA 16000 14000
2012 21 MA 16000 14000
2012 24 MA 16000 15000
2012 27 MA 16000 17000
In other words, my REVENUE stops developing at a DEV_AGE of 12 (there are no rows in the REVENUE table beyond a DEV_AGE of 12), but I want every DEV_AGE beyond 12 to equal what the REVENUE was at 12 in the final table.

Answer 1

Here is an approach that uses window functions to calculate the revenue for age 12 and then logic to assign it:
select acc_yr, dev_age, state,
(case when dev_age > 12 then rev12 else revenue end) as revenue, loss
from (select l.acc_yr, l.dev_age, l.state, r.revenue, l.loss,
max(case when l.dev_age = 12 then r.revenue end) over (partition by l.acc_yr, l.state) as rev12
from loss l left join
revenue r
on l.acc_yr = r.acc_yr and l.dev_age = r.dev_age and l.state = dev.state
) lr;