It isn't. It's just incorrectly spelled in the question.

It works if your database uses one of the _SC collations. In that case, NCHAR accepts arguments up to 0x10ffff. It's an easy one to miss, and if your database...

Backslashes do have a tendency to disappear when moved between environments...

I must admit I've never seen the construct "IS DISTINCT FROM NULL" before, and initially I thought that must be the error, but it's actually legit. My next thought was...

Yep. Not touching this quite yet...

Yep. Not touching this quite yet...

What is the precision, scale, and length for an int variable?

The correct answers are:
10, 4, 0
Explanation
The precision of an int is 10, the length is 5, and...

The question was "How many unique values are returned?", not "how many rows are returned that contain unique values?". So, other than the minor quibble with the missing column from the...

Nothing, really, seeing as there is not [total] column in the table...

If you're just seeing if the customerid exists in the other table shouldn't this be an EXISTS clause? No need to return the results of the subquery just see...

There are all sorts of wrong with that code, but only one that actually stops it from executing.

Nice one!

With this set of original data, it's not really doing much, since all the ID-s are different, so the inner SELECT will effectively yield the original table, and the...

Except you didn't really. Somehow you changed what I answered, from what is now the correct answer to the answer initially marked as correct.

I'm thinking both the answer and explanation got a bit lost in translation...

You can remove the TOP  or the WITH CHECK option. Either will do the trick, as well as removing both.