T-SQL, Find numeric values - sql-server

Question

I did this quiz on http://www.sql-ex.ru/, question 35 to be exact.
The question is as follows:
In Product table, determine the models which consist only of digits or only of latin letters (A-Z, case insensitive).
Result set: model, type of model.
And I gave the correct answer which is:
SELECT model,
type
FROM Product
WHERE Model NOT LIKE '%[^A-Z]%'
OR Model NOT LIKE '%[^0-9]%'
Now my question is why do I need double negations to make it work.
If I rewrite the code to:
SELECT model,
type
FROM Product
WHERE Model LIKE '%[A-Z]%'
OR Model LIKE '%[0-9]%'
I get the wrong answer:
Your query returned the correct dataset on the first (available) database, but it returned incorrect dataset on the second checking database.
* Wrong number of records (more by 37)
How come that the first example of code gives the correct answer while the second example doesn´t?
I have tried to find answer but no luck. Grateful for an explanation.

Answer 1

Where Model LIKE '%[A-Z]%' Or Model LIKE '%[0-9]%'
Matches rows where Model contains at least one alpha numeric character.
This does not exclude in any way those values that contain mixed alphanumeric and non-alphanumeric characters.
e.g. ~A#- would pass because of the presence of the A
Moreover your correct query matches either
'%[^A-Z]%': those strings which do not contain any non letters (i.e. consist of only letters or are empty)
'%[^0-9]%': those strings which do not contain any non digits (i.e. consist of only digits or are empty).
This is not handled at all in your second attempt and a mixed string of letters and digits would be accepted by that.
I would use your first attempt but if you were determined to avoid the double negative you could use
SELECT model
FROM Product
WHERE Model LIKE REPLICATE('[A-Z]', LEN(Model))
OR Model LIKE REPLICATE('[0-9]', LEN(Model))