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The McGraw Hill Companies, 2001
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the relationship set borrower is total In contrast, an individual can be a bank customer whether or not she has a loan with the bank Hence, it is possible that only some of the customer entities are related to the loan entity set through the borrower relationship, and the participation of customer in the borrower relationship set is therefore partial
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23 Keys
We must have a way to specify how entities within a given entity set are distinguished Conceptually, individual entities are distinct; from a database perspective, however, the difference among them must be expressed in terms of their attributes Therefore, the values of the attribute values of an entity must be such that they can uniquely identify the entity In other words, no two entities in an entity set are allowed to have exactly the same value for all attributes A key allows us to identify a set of attributes that suf ce to distinguish entities from each other Keys also help uniquely identify relationships, and thus distinguish relationships from each other
231 Entity Sets
A superkey is a set of one or more attributes that, taken collectively, allow us to identify uniquely an entity in the entity set For example, the customer-id attribute of the entity set customer is suf cient to distinguish one customer entity from another Thus, customer-id is a superkey Similarly, the combination of customer-name and customer-id is a superkey for the entity set customer The customer-name attribute of customer is not a superkey, because several people might have the same name The concept of a superkey is not suf cient for our purposes, since, as we saw, a superkey may contain extraneous attributes If K is a superkey, then so is any superset of K We are often interested in superkeys for which no proper subset is a superkey Such minimal superkeys are called candidate keys It is possible that several distinct sets of attributes could serve as a candidate key Suppose that a combination of customer-name and customer-street is suf cient to distinguish among members of the customer entity set Then, both {customer-id} and {customer-name, customer-street} are candidate keys Although the attributes customerid and customer-name together can distinguish customer entities, their combination does not form a candidate key, since the attribute customer-id alone is a candidate key We shall use the term primary key to denote a candidate key that is chosen by the database designer as the principal means of identifying entities within an entity set A key (primary, candidate, and super) is a property of the entity set, rather than of the individual entities Any two individual entities in the set are prohibited from having the same value on the key attributes at the same time The designation of a key represents a constraint in the real-world enterprise being modeled Candidate keys must be chosen with care As we noted, the name of a person is obviously not suf cient, because there may be many people with the same name In the United States, the social-security number attribute of a person would be a
Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
I Data Models
2 Entity Relationship Model
The McGraw Hill Companies, 2001
2
Entity-Relationship Model
candidate key Since non-US residents usually do not have social-security numbers, international enterprises must generate their own unique identi ers An alternative is to use some unique combination of other attributes as a key The primary key should be chosen such that its attributes are never, or very rarely, changed For instance, the address eld of a person should not be part of the primary key, since it is likely to change Social-security numbers, on the other hand, are guaranteed to never change Unique identi ers generated by enterprises generally do not change, except if two enterprises merge; in such a case the same identi er may have been issued by both enterprises, and a reallocation of identi ers may be required to make sure they are unique
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