Compound keys use multiple attributes to uniquely identify an entity in a table. They are used for complex information structures and should only be used when a single attribute key is not enough. Foreign keys link tables together using the same key combination.
In organizational database technology, a “key” is a particular attribute in a table used to uniquely identify a specific entity. A compound key uses two or more attributes to achieve the same goal; only when in tandem do these elements mean a “match” with the target element. Composite keys are typically used for tables whose information structure is too complex to work using a single key. Another requirement of compound keys is that none of the individual attributes chosen for a compound key can be treated as a simple single-attribute key by itself.
One of the most basic examples of a database table that requires a composite key is an employee or student directory. Imagine that the table contains a list for “First Names” and another list for “Last Names”. By themselves, neither the first or last names of a particular student or employee would be sufficient to guarantee a unique identification, since it is possible for two or more people in the database to have matching first or last names. When paired with a composite key, using both the “First Name” and “Last Name” entries to identify a particular student or employee, the likelihood that each first and last name combination is unique increases dramatically. Therefore, only a key composed of a first and last name is sufficient to uniquely identify a particular student or employee.
Remember that compound keys should only be used if a single attribute “simple” key is not enough. Continuing the previous example, if each student or employee also had a unique identification number, it would make the need for composite keys entirely unnecessary. If a particular student or employee could be identified by her number alone, there would be no need to worry about either first or last name; the only attribute that matters is the identification number.
While each individual attribute in a compound key is itself non-unique, any attribute can still be selected as a foreign key. A foreign key links two or more tables in a database. For example, consider a university database where there is a separate table for student information and teacher information, and the combination of ‘First Name’ plus ‘Last Name’ is used as a composite key in the student table. The same key combination consisting of “First Name” plus “Last Name” can then be used to link the student and teacher tables together, providing a gateway between the two.
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