Unique Key

Unique Key
1. A unique key can allow null values. (You can define a unique key on columns that allow nulls.)
2. Each table can have multiple unique keys.
3. On some RDBMS A unique key automatically generates a non-clustered table index by default.

Read more about Unique Key:  Defining Unique Keys, Surrogate Keys, Alternate Key

Famous quotes containing the words unique and/or key:

    When each thing is unique in itself, there can be no comparison made.... There is only this strange recognition of present otherness.
    —D.H. (David Herbert)

    It so happened that, a few weeks later, “Old Ernie” [Ernest Hemingway] himself was using my room in New York as a hide-out from literary columnists and reporters during one of his rare stopover visits between Africa and Key West. On such all-too-rare occasions he lends an air of virility to my dainty apartment which I miss sorely after he has gone and all the furniture has been repaired.
    Robert Benchley (1889–1945)