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When more than one column is combined to form a unique key, their combined value is used to access each row and maintain uniqueness. Values are not combined, they are compared using their data types.

When a column or set of columns is defined as unique to the database management system, the system verifies that each set of value(s) is unique before assigning the unique constraint. After the column(s) are defined as unique, an error will occur if an insertion is attempted with values that already exist.

http://en.wikipedia.org/wiki/Unique_key

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