EDITED after reading the MSDN forum link from the comment, very interesting.
Regardless of isolation level, two users cannot update a single page simultaneously, nor can any user read a partially updated page. Just imagine how SQL Server would deal with a page where the header says Col3 starts at byte 17. But it really starts at byte 25, because that part of the row hasn't been updated yet. There's no way a database could handle that.
But for rows larger than 8k, multiple pages are used, and that makes a half-updated column possible. Copied from the MSDN link (in case the link breaks), start this query in one window:
if object_id('TestTable') is not null
drop table TestTable
create table TestTable (txt nvarchar(max) not null)
insert into TestTable select replicate(convert(varchar(max),
update TestTable set txt=replicate(convert(varchar(max),
This creates a table and then updates it with a string of 100.000x the same character. While the first query is running, start this query in another window:
while 1=1 begin
if exists (select * from TestTable (nolock) where left(Txt,1) <> right(Txt,1))
The second query stops when it reads a column that is half updated. That is, when the first character is different from the last. It will finish quickly, proving that it is possible to read half-updated columns. If you remove the
nolock hint, the second query will never finish.
Surprising result! A half-updated XML column might break a
(nolock) report, because the XML would be malformed.