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:
“An absolute can only be given in an intuition, while all the rest has to do with analysis. We call intuition here the sympathy by which one is transported into the interior of an object in order to coincide with what there is unique and consequently inexpressible in it. Analysis, on the contrary, is the operation which reduces the object to elements already known.”
—Henri Bergson (1859–1941)
“All meanings, we know, depend on the key of interpretation.”
—George Eliot [Mary Ann (or Marian)