Question

Earlier today, I asked this question and got the answer I was looking for. Now I have a follow-up question:

What I want:

I want the MERGE to compare each column value, per row, in the target table against the corresponding value in the source table, and make any updates based on the logic separated by OR in the WHEN MATCHED AND block.

I am afraid that the code I’ve written (pictured below) will make the updates listed in the THEN UPDATE SET block if any of the logic separated by OR in the WHEN MATCHED AND block is true.

If my hunch is correct, do you have any suggestions for how to re-write the code to have it behave like I want it to behave?

Answer 1

Not having your data, and not wanting to re-type your query from an image, I created a sample that I think demonstrates what you want:

create table t (ID int not null,Col1 int null,Col2 int null)
create table s (ID int not null,Col1 int null,Col2 int null)

insert into t(ID,Col1,Col2) values (1,1,null),(2,null,2)
insert into s(ID,Col1,Col2) values (1,3,4),(2,5,6),(3,7,8)

;merge into t
using s
on t.ID = s.ID
when not matched then insert (ID,Col1,Col2) values (s.ID,s.Col1,s.Col2)
when matched then update
set Col1 = COALESCE(t.Col1,s.Col1),
Col2 = COALESCE(t.Col2,s.Col2)
;
select * from t

Result:

ID          Col1        Col2
----------- ----------- -----------
1           1           4
2           5           2
3           7           8

Where the key is to use COALESCE to avoid updating a column value if it already has one (which I think is what you’re trying to achieve)