Determining Candidate Keys
The previous example only illustrates the definition of a candidate key and not how these are determined in practice. It is important to determine all superkeys, which is especially difficult if the relation represents a set of relationships rather than a set of entities. Therefore it is often useful to attempt to find any "forgotten" superkeys by also determining the functional dependencies. We can derive more superkeys by applying the following rule:
- if S is a superkey and X→Y a functional dependency
- then (S ∖ {Y}) ∪ {X} is also a superkey, where '\' is the set difference.
Consider for example the relation
- Marriage (Husband, Wife, Date)
for which it will trivially hold that
- {Husband, Wife, Date}
is a superkey. If we assume that a certain person can marry at most once on a given date then this implies the functional dependencies:
- {Husband, Date} → Wife
- {Wife, Date} → Husband
In this case, applying the above rule leads to the derivation of the superkeys {Husband, Date} and {Wife, Date} respectively.
Read more about this topic: Candidate Key
Famous quotes containing the words determining, candidate and/or keys:
“I esteem it the happiness of this country that its settlers, whilst they were exploring their granted and natural rights and determining the power of the magistrate, were united by personal affection. Members of a church before whose searching covenant all rank was abolished, they stood in awe of each other, as religious men.”
—Ralph Waldo Emerson (1803–1882)
“If we should swap a good library for a second-rate stump speech and not ask for boot, it would be thoroughly in tune with our hearts. For deep within each of us lies politics. It is our football, baseball, and tennis rolled into one. We enjoy it; we will hitch up and drive for miles in order to hear and applaud the vitriolic phrases of a candidate we have already reckoned we’ll vote against.”
—Federal Writers’ Project Of The Wor, U.S. public relief program (1935-1943)
“McCoy: That shark’s been following us ever since the surgeon died, waiting for the burial. Couldn’t I have a musket to shoot it, sir?
Fletcher Christian: Take the deck, McCoy. I’ll get the keys to the arms chest.
McCoy: Get two muskets, sir. I’d like to shoot that shark on board.”
—Talbot Jennings (1896–1985)