Grammars about a nursery rhyme "Old King Cole"

In the following nursery rhyme,

Old King Cole was a merry old soul,
And a merry old soul was he;
He called for his pipe,
And he called for his bowl,
And he called for his fiddlers three.
Every fiddler had a fiddle,
And a very fine fiddle had he.
Tweedle dee, tweedle dee,
Tweedle dee, tweedle dee,
Tweedle dee, tweedle dee,
Went the fiddlers three,
Oh there's none so rare
As can compare,
With King Cole and his fiddlers three.

Please help me to solve my problems,

1. How many meanings are there of the phrase "so ... as ..."?And what "part of speech" are they? Conjunctions? Adjectives? Pronouns?

2. If we omit these words "so rare as", maybe the meaning is much more clear?

"There's none (who) can compare with King Cole ...."

But the grammar will be wrong if there isn't a "who" added. If we don't take off these three words "so rare as", the grammar of the original sentence seems strange."There's none so rare as can compare, with King Cole and his fiddlers three."What in the world is the meaning of this sentence?

Thank you very much for your reply!!