Here is what is wrong with last week's picture painted by Martijn Evers in Kinds of Keys: On the Nature of Key Classifications.
"Many data and information modelers talk about all kinds of keys (or identifiers. I'll forego the distinction for now). I hear them they talk about primary keys, alternate keys, surrogate keys, technical keys, functional keys, intelligent keys, business keys (for a Data Vault), human keys, natural keys, artificial keys, composite keys, warehouse keys or Dimensional Keys (or Data Warehousing) and whatnot. Then a debate rises on the use (and misuse) of all these keys ... The foremost question we should actually ask ourselves: can we formally disambiguate kinds of keys (at all)?"I, of course, sympathize, as I've deplored the misuse and abuse of terminology in the industry for longer than I care to remember. This is not unique to keys, it's a general absence of foundation knowledge. The answer is yes, we can disambiguate--the key to keys is understanding what in the real world they represent and what database function they fulfill. Things are much simpler than the mindless industry jargon.
A key is a set of one or more relation attributes that uniquely identify tuples, which represents facts about entities in the database. Uniqueness (key) constraints on the relation for every key enforces entity integrity (i.e., there is no duplicate representation of facts about any entity).
"Of all kinds of key, the primary key and the surrogate key gained the most discussion.If we take a look at the relational model we only see candidate keys a minimal set of one or more attributes that are unique for each tuple in a relation--no other formal distinction is possible. When we talk about different kinds of keys we base our nomenclature on properties and behavior of the candidate keys. We formally do not have a primary key, it is a choice we make and as such we might treat this key slightly different from all other available keys in a relation. The discussion around primary keys stems more from SQL NULL problems, foreign key constraints and implementing surrogate keys."Codd mandated a primary key (PK) for every relation with multiple keys, mainly for referential purposes (i.e., as target for references by other relations). Obviously, a single key is the PK, but when there are multiple keys, they are candidates and one is selected as PK. The choice is, indeed, not formal (which is why Date downgraded Codd's mandate to just "a good idea"; I would not advise to skip it), but is not arbitrary either: for practical reasons, stability and simplicity are desirable PK qualities.
Entity Properties and Natural Keys
Natural keys (NK) represent in the database entity-defining identifiers (EDI): a set of one or more observable/assertable properties inherent to all entities that defines and uniquely identifies each (e.g., DNA for people, ownership and a set of books for companies). As such, they are meaningful to users and are used for logical data access. EDI's and, therefore, NK's are immutable by definition--they change only if the entities themselves change (e.g., companies involved in mergers, acquisitions, spinoffs)--but tend to be complex (i.e., combinations of attributes), or have complex representations (e.g., DNA). They don't make practical PK's.
Entity Names and Surrogate Keys
When no stable and simple EDI can be identified or explicitly specified in the real world, entities are usually assigned a unique name to substitute for the complex or implicit EDI (e.g., SSN, S/N, patient id), such that all descriptive properties and other names are functionally dependent on it. Entity names are represented in the database by surrogate keys (SK) and all other attributes and keys are functionally dependent on it.
SK's have nothing to do with SQL and NULL's--SQL tables with NULL's are not relations and, therefore, outside the RDM, where all bets are off.
"If we treat surrogate keys as an arbitrary candidate key, we actually decrease the normalization of a data model from, say, 3NF to 2NF. This has led to the special and strict treatment we have for surrogate keys as to prevent this from leading to unwanted issues. Surrogate keys should be seen as not an extra key but as a surrogate for an existing key whose semantics and behavior is strictly regulated by the DBMS system."As its name implies, SK's are not keys--they do not represent real world properties--but key substitutes that are added to other keys, if any. Since all attributes are functionally dependent on a SK by definition, a surrogate PK does not decrease normalization.
"Business Keys on the other hand are independent keys that are not designated as a surrogate key whose familiarity, uniqueness, visibility and scope span an entire enterprise. Human keys for example are actually not keys in the strict sense, but are non-enforceable candidate keys that heuristically identify tuples, especially those related to real world entities. They are related to the process of relating (UoD) entities to tuples. E.g. using full name and full address to identify and relate a real world person to a tuple representing that person in a database and uniquely identifying that person by a Person ID."NK's ensure entity integrity and are used for meaningful logical data access. SK's serve as simple and stable PK's for referential purposes. What in the real world do "business keys", or "human keys" represent, what database functions do they fullfill that keys/NK's/PK's and SK's do not and what are their desirable qualities?
Note: An "intelligent key" (also referred to as column overload) is an attribute defined on a non-simple domain with meaningful components. This is hardly intelligent practice (see The First Normal Form in Theory and Practice, Parts 1-3).
Request to my readers (particularly the regular ones): My current writings are in large part dedicated to making accessible to practitioners David McGoveran's interpretation of Codd's RDM, with refinements, extensions and corrections he believes Codd would have ended up with had he lived long enough. In some important aspects there is divergence from the current understanding of the RDM (well, whatever little understanding there is). David will introduce the formal theory in his forthcoming book, LOGIC FOR SERIOUS DATABASE FOLKS. In my writings I strive to make the theory and its practical implications accessible to the thinking data professional--material is not available anywhere else. Please disseminate it by linking to my posts, writings and publications on as many of the web and social media sites for whose audience may deem it of interest. Thanks.