secondary key
A key is the field (the key field) within a record used to identify the record, for example, a bank account number identifies a customers account. The key can be used for finding the record within a file or as the sort key for sorting a file into order. Most data files will have a primary key, which is unique and used to identify the record. If the records in a data file can be accessed sequentially the records will be accessed in key field order, which is the order they will be in when they have been sorted using the key field. Data files may also have secondary keys, which enable the file to be accessed in a different order. Composite keys, made up of more than one field, can be used to sort a file.
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