SQL Get First Non Null Value By Date From A Table - sql

Question

Im trying a CASE expression, something like this:
LEFT OUTER JOIN table_1 AS T1
ON T2.common_id = T1.common_id
CASE WHEN T1.column_1 IS NOT NULL
THEN T1.column_1
WHEN T1.column_1 IS NULL
THEN get first value from T1.column that is not null by date
ELSE 0
END
Its the logic for that 6th line which I have written out what I want that I can't quite get right.
THEN get first value from T1.column that is not null by date
I have been looking at the FIRST_VALUE function but can't quite figure it out correctly. But there are maybe other ways to get it to work.

Answer 1

you can to find FIRST_VALUE that is not null with use this query
First use order by date for find date is null
after get value FIRST_VALUE date with FIRST_VALUE
select *,FIRST_VALUE(date) over(partition by T1.common_id order by case when date is not null then 1 else 2 end)
from #table_2 T2
LEFT OUTER JOIN #table_1 AS T1
ON T2.common_id = T1.common_id
You can to insert the basic data with the following codes
create table #table_1 (id int,common_id int,date datetime)
create table #table_2 (id int,common_id int,name varchar(100))
insert into #table_1(id,common_id,date)
select 1,1,'2015-05-24' union select 2,1, null union select 3,1, '2014-09-01' union
select 4,4,null union select 5,4, '2019-08-05' union select 6,4, '2000-09-07' union
select 7,7,null union select 8,7, '2019-08-05' union
select 9,12,'2019-08-06' union select 10,12, '2019-08-05' union
select 11,18,'2019-08-06' union select 12,19, '2019-08-05'
insert into #table_2(id,common_id,name)
select 1,1,'a' union select 2,1, null union select 3,1, 'b' union
select 4,4,'k' union select 5,4, 't' union select 6,4, 'c' union
select 7,7,'aaa' union select 8,7, 'sada' union
select 9,12,'44dd' union select 9,12, '44'