SQL Server 2022

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create table a (ID int, Count_A int);
insert into a values
 (1,           1)
,(2,           2)
;
create table b(ID int, Count_B int);
insert into b values
 (1,           3)
,(3,           4)
;
create table c(ID int, Count_C int);
insert into c values
 (2,           5)
,(4,           6)
;
6 rows affected

select coalesce(a.id,b.id,c.id) id, a.count_a, b.count_b, c.count_c
from a
full outer join b
    on a.id = b.id
full outer join  c  
    on a.id = c.id
    or b.id = c.id 


id
count_a
count_b
count_c


1
1
3
null

2
2
null
5

3
null
4
null

4
null
null
6


select coalesce(a.id,b.id,c.id) id, a.count_a, b.count_b, c.count_c
from a
full outer join b
    on a.id = b.id
full outer join  c  
    on isnull(a.id, b.id) = c.id


id
count_a
count_b
count_c


1
1
3
null

2
2
null
5

3
null
4
null

4
null
null
6

id	count_a	count_b	count_c
1	1	3	null
2	2	null	5
3	null	4	null
4	null	null	6

id	count_a	count_b	count_c
1	1	3	null
2	2	null	5
3	null	4	null
4	null	null	6