Having a hard time building an aggregate SQL query - sql

Question

I am new at SQL and have a pretty good knowledge of basic stuff but I am stuck with my request.
My request gets me te following table (except for the last column on the right end side):
Team
Variable
Date
Value
Column_I_need_to_add
A
aa
2022/05/01
100
0
A
aa
2022/06/01
25
0
A
aa
2022/07/01
580
0
A
ad
2022/08/01
50
605
B
aa
2021/05/01
75
0
B
aa
2021/06/01
110
0
B
aa
2021/07/01
514
0
B
ad
2021/08/01
213
624
What I cannot turn my head around, is how to code for the last column that fills rows for the ad variable by summing values of the aa variables of the same team but only for the two months prior to the date of the ad variable.
Here is the script I have so far, that gets me the first four columns:
SELECT
team.Team,
Var.Variable,
TO_DATE(Var.Year||'-'||LPAD(Var.Month,2,'00')||'-'||'01','YYYY-MM-DD')AS Date ,
Var.value
FROM table1 as Var
join table2 as team
on Var.code=team.code
---This last join with table3 is only there to add other columns that are not relevant to this problem.
---join table3 as detail_var on Var.variable=detail_var.code_var

Answer 1

I was not content with the previous answer, with OUTER APPLY, as understood from further reading. So had to do a bit of further grinding and this is what I came up with (Now for Postgres 13).
It is cleaner and does the job in a conciser fashion. I've also added a FIDDLE LINK. If you want to see the previous answer please look at the edit versions.
SELECT
team.Team
,var.Variable
,var.Date
,var.value
,CASE
WHEN var.Variable='ad' THEN
(SELECT sum(value) FROM table1
WHERE
(TO_DATE(Year||'-'||LPAD(Month::varchar(2),2,'0')||'-'||'01','YYYY-MM-DD')
BETWEEN (var.Date - INTERVAL '2 month') AND var.Date)
AND Variable = 'aa'
AND code = var.code)
ELSE null
END as past2monthsValue
FROM (
-- this sub query to change Year & Month to Date Type Value
-- this Date Type Value (Date) will be used to compare dates
-- (var.Date) in the above sub-query
SELECT
code,
Variable,
TO_DATE(Year||'-'||LPAD(Month::varchar(2),2,'0')||'-'||'01','YYYY-MM-DD') AS Date,
value
FROM table1
) var
JOIN table2 AS team ON var.code=team.code