Question

I am trying to resolve t-sql exercise
I need to update first table with values from second by joining by id. If I can not join then use value from default ID (default iD is the Id that is null)

please run it to see it

declare @t as table (
    [id] INT
    ,val int
)

insert into @t values (null, null)
insert into @t values (2, null)
insert into @t values (3, null)
insert into @t values (4, null)

declare @t2 as table (
    [id] INT
    ,val int
)

insert into @t2 values (null, 11)
insert into @t2 values (2, 22)
insert into @t2 values (3, 33)


select * from @t
select * from @t2

update t
set t.val = t2.val
from @t as t join @t2 as t2
    on t.id = t2.id
        or 
        (
            (t.id is null or t.id not in (select id from @t2))
            and t2.id is null
        )




select * from @t

here is result

--@t
id      val
---------------
NULL    NULL
2       NULL
3       NULL
4       NULL

--@t2
id      val
---------------
NULL    11
2       22
3       33

--@t after update
id      val
---------------
NULL    11
2       22
3       33
4       NULL

how to make val in last row equal 11?

4       11

Answer 1

try this for the update…

update t
set t.val = t2.val
from @t as t join @t2 as t2
    on t.id = t2.id
        or 
        (
            (t.id is null or not exists (select * from @t2 where id = t.id))
            and t2.id is null
        )

Problem is with not in operator and null values. This would also work…

update t
set t.val = t2.val
from @t as t join @t2 as t2
    on t.id = t2.id
        or 
        (
            (t.id is null or t.id not in (select id from @t2 where id is not null))
            and t2.id is null
        )