So imagine I have a relation like this:

r(A, B, C, D, E)

Where A is the primary key and C is unique. I'm having doubts on how the functional depedencies work, I know since A is the primary key every other attribute depends on it but I don't know what to do with the unique attribute. Does it work the same way as the primary key? Does C depend on A (A-->C)?

I've also seen another case where I have a relation like the one above but with a unique signature with two attributes on it, like this:

unique(C, D)

And I also don't know the meaning of a unique with two attributes instead of a unique declaration for each attribute and how it will affect the functional depedencies.

Thank you for your time and for your responses in advance.