MSSQL Selektieren einer bestimmten Zeile

begin
   for rec in (select b.id as theUpdateID, a.name, a.vorname, count(*) over( partition by a.b_id order by b.id Rows Unbounded Preceding ) as rn
                 from test_b b
                      join test_a a on (a.b_id = b.id)
                order by b.id) loop
         if rec.rn = 1 then
            update test_b set name1 = rec.vorname || ' ' || rec.name where id = rec.theUpdateID;  
         end if;
         if rec.rn = 2 then
            update test_b set name2 = rec.vorname || ' ' || rec.name where id = rec.theUpdateID;  
         end if;
         if rec.rn = 3 then
            update test_b set name3 = rec.vorname || ' ' || rec.name where id = rec.theUpdateID;  
         end if;
   end loop;
end;
/

declare updCursor Cursor for
	select b.id as theUpdateID
		 , a.name, a.vorname
		 , ROW_NUMBER() over( partition by a.b_id order by a.id) as rn
	  from tab_b b
			left join tab_a a on (a.b_id = b.id)
		 order by a.b_id;
declare @updID VarChar( 30 );
declare @name VarChar( 30 );
declare @vorname VarChar( 30 );
declare @rn VarChar( 3 );
open updCursor;
fetch next from updCursor into @updID, @name, @vorname, @rn;
while @@FETCH_STATUS = 0
	begin
		declare @stmt NVarChar( max );
		set @stmt = 'update tab_b set name' + @rn + ' = ''' + @vorname + ' ' + @name + ''' where id = ' + @updID;  
		print @stmt;
		exec sp_executesql @stmt;
		
		fetch next from updCursor into @updID, @name, @vorname, @rn;
	end;
close updCursor;
deallocate updCursor;

updID   TEST_A.id%type;