Secondary key

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A secondary key is an alternative search key that, in addition to the primary key , can be used in a database system as an additional search criterion for finding one or more data records. Like any search key, a secondary key can comprise one or more attributes .

In contrast to a primary key , a secondary key is not necessarily unique. It can therefore return several data records as the result of a search.

Foreign key as secondary key

In order to efficiently carry out the search using a secondary key, it makes sense to index the attributes of the secondary key using a so-called secondary index. Secondary indices play a special role in connection with foreign keys , which refer to the primary key of another relation or the same relation. To efficiently carry out a link operation that locates the related data records of two relations on the basis of such a primary-foreign key relationship, it is helpful if the associated link partners in the foreign key can be found using a secondary index based on the attribute values ​​in the primary key. For each attribute value in the primary key, all data records are searched that have the same attribute values ​​in the associated foreign key.

example

An address database is searched for the name Müller as a search criterion. There are numerous hits because the name Müller is a common surname.

literature

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