Database - Define alternate key

Define alternate key.

There can be a key apart from primary key in a table that can also be a key. This key may or may not be a unique key. For example, in an employee table, empno is a primary key, empname is a alternate key that may not be unique but still helps in identifying a row of the table.

Define alternate key.

An alternate key is similar to a primary key. It accepts null values; where as the primary key does not. The null values can be submitted to the attribute in a tuple.
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Candidate and Alternate keys
Any attribute that uniquely defines a row in a table is a candidate for the primary key.so that attribute would be called a candidate key based on familiarity,greater usage and so on.

Any attribute that is a candidate of the primary key but is not a primary key is called an alternate key.
Esther 03-22-2014