If I am writing a sentence where both a noun and an adjective are used as modifiers, shall I write first the adjective, or the noun?
It's a nice C code snippet.
(?) It's a C nice code snippet.
Is the same "rule" valid for the following sentence?
Reserved SQL keywords are written in uppercase.
SQL reserved keywords are written in uppercase.
In all of the cases that you mention above, the noun in apposition should come nearest to the head noun.
It's a nice C code snippet.
Reserved SQL keywords are written in uppercase.
This example is completely ungrammatical:
! It's a C nice code snippet. [Incorrect]
Your second example is grammatical both ways, but there is a subtle difference in meaning, because the sentences are parsed in different ways:
{Reserved {SQL keywords}} are written in uppercase.
{{SQL reserved} keywords} are written in uppercase.
Because nouns must appear closer to the head than adjectives, the entire phrase "SQL reserved" in the second example is parsed as a noun phrase modifying "keywords".
In your example "It's a nice C code snippet", the only modifier is the adjective "nice", and that modifies the noun "C code snippet". In turn, "C code snippet" is a compound noun made by combining the noun "C code" and the noun "snippet". In turn, "C code" is a compound noun made by combining the noun "C" and the noun "code".
That is, using catagory labels with left square brackets, we have the noun phrase "a nice C code snippet" with the structure
[NP [Det a] [Adj nice] [N [N [N C] [N code]] [N snippet]]]
Since usually compound words have most stress on the first element while phrases have more stress on the last element, the above structure for this noun phrase predicts that "nice" will have less stress than the following noun "C code snippet", while "C" has more stress than "code", and "C code" has more stress than "snippet". I believe this is correct.
The first parts of compound nouns are sometimes said to be adjectives, but although the interpretation of the compound may suggest this, I think it's wrong. Grammatically, in these examples at least, "C" and "C code" are nouns. You can't have compound nouns containing an adjective modifying a following noun; that would be a phrase rather than a word. Your example "! It's a C nice code snippet. [Incorrect]" is bad because the adjective "nice" cannot go inside the compound noun "C code".
