问题
Functional dependencies are the attributes that their values are determined in a unique way by another attribute.Given that, can a multivalued attribute be dependent upon a primary key?
回答1:
"FDs are the attributes that their values are determined in a unique way by another attribute" is unintelligible. Find a way to say it correctly or how can you understand it?
An attribute (or set of attributes) is functionally determined by a set of attributes.
There is no such thing as a "multivalued attribute" in a relation. A tuple has an attribute value for each attribute name. (Maybe you mean, a set of attributes is being determined? Maybe you mean, a multi-valued dependency?) If you have an attribute that you consider to contain multiple parts, ie you want to generically query about the parts without using operators with parameters of their types, then it is usually good design to have a separate table with attributes for those parts. But that's not addressed by normalization. Any value can be considered to have multiple parts in multiple ways and it is your application/queries that determine when you stop making tables whose attributes are the values of parts of other values and just have an attribute for a value. Similarly, if you have a bunch of attributes that play a similar role (often with similar names) then it is usually good design to have a separate table with just one attribute for the role. But that's not addressed by normalization.
Candidate keys matter to FDs, MVDs, JDs and normalization. PKs don't. You can pick one CK as "the PK" but its primariness is irrelevant to the relational model. It might be relevant to some information modeling method or product.
Superkeys are sets of columns that determine every column. Since every set of attributes always determines the attributes in it, superkeys are sets of columns that determine every other column. CKs are superkeys that contain no smaller superkey. (So CKs are sets of columns that are unique but contain no smaller set of columns that are unique.)
You don't know all the CKs until you find all the FDs. But you might know that a particular set of attributes is unique and has no smaller unique set, so that you know that it is a CK and you can call it "the PK". (Eg an id attribute in a relation variable that can have more than one row.)
can a multivalued attribute be dependent upon a primary key?
- Every attribute is dependent on every CK by definition of CK. So every attribute is dependent on every PK by definition of PK.(But you must clarify what you mean by "multivalued attribute" and "dependent".)
来源：https://stackoverflow.com/questions/32144501/can-a-multivalued-attribute-have-a-primary-key