SQL Server 2019

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create table comments(
  id int primary key,
  name varchar(100),
  comment varchar(100)
)
create procedure p_merge (@j nvarchar(max))
as
begin

  merge into comments as t
  using (
    select
      max(case [key] when 'id' then value end) as id,
      max(case [key] when 'name' then value end) as name,
      max(case [key] when 'comment' then value end) as comment,
      max(case [key] when 'id' then flag end) as id_upd,
      max(case [key] when 'name' then flag end) as name_upd,
      max(case [key] when 'comment' then flag end) as comment_upd
    from (
      select j.*, 1 as flag
      from openjson(@j) as j
    ) as p
  ) as s
    on s.id = t.id
  when matched then update
    set name = case when s.name_upd = 1 then s.name else t.name end,
        comment = case when s.comment_upd = 1 then s.comment else t.comment end
  when not matched then insert
    values(s.id, s.name, s.comment)
  ;

end;
exec p_merge @j = N'{ "id": 1, "name": "Name 1", "comment": "Test comment" }';
select *
from comments;


id
name
comment


1
Name 1
Test comment


Warning: Null value is eliminated by an aggregate or other SET operation.


exec p_merge @j =N'{ "id": 1, "name": "Name 2" }';
select *
from comments;


id
name
comment


1
Name 2
Test comment


Warning: Null value is eliminated by an aggregate or other SET operation.


exec p_merge @j =N'{ "id": 1, "comment": null }';
select *
from comments;


id
name
comment


1
Name 2
null


Warning: Null value is eliminated by an aggregate or other SET operation.