What do you call something that's not the primary key?

I've been working with a group who have built a system where they've coined a few new terms in the process, and I wanted to know if there's something more standardized they should be calling it.

Basically, they have tables which hold versioned info, so the unique key for the table is the identifier for the document being tracked, plus the record number (although, the record number is auto-numbered, so it's unique in itself). They refer to the fields that make up the document's identifier (it changes for each table), as the 'prime key' ... but the fields both individually and collectively (eg, if it has fields A, B and C to get a unique identifier, they might interchangable refer to A as being 'a prime key' and A/B/C as being 'the prime key'

If they had a normalized database, this would be an obvious candidate for a foreign key -- but it's not normalized, so it's not a reference to a different table.

Is there some name for this concept -- an identifier to select some concept that's not what the table is based on, but which isn't a foreign key?

update: it's not an alternate key, as it doesn't identify a unique record within the table; it identifies a group of records. Take the folowing table:

id   document_id   version_no
--   -----------   ----------
 1             1            1
 2             2            1
 3             1            2
 4             2            2
 5             2            3

So we're tracking metadata for 5 objects, which are two different files, one of which has been updated once (2 versions/editions of it), and one which has been updated twice.

We have the following:

• candidate keys : (id) or (document_id, version_no)

• natural key : (document_id, version_no)

• surrogate key : (id)

• primary key : (id, document_id) (I didn't select it)

... if document_id were a relationship to another table, it'd be a foreign key, but 'foreign key' typically also infers a constraint (ie, the value has to exist in some other table bef

User terms surrogate key and natural key for primary key as a variant.


Surrogate Key:


Surrogate keys are keys that have no
“business” meaning and are solely used
to identify a record in the table.
Such keys are either database
generated (example: Identity in SQL
Server, Sequence in Oracle,
Sequence/Identity in DB2 UDB etc.) or
system generated values (like
generated via a table in the schema).


Natural Key:


Keys are natural if the attribute it
represents is used for identification
independently of the database schema.
What this basically means is that the
keys are natural if people use them
example: Invoice-Numbers, Tax-Ids, SSN
etc.

Surrogate Keys vs Natural Keys for Primary Key

StackExchange Database Administrators Q#534, answer score: 7