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 the object is perceived as particular and unique and not merely the member of a family, when it appears independent of any general notion and detached from the sanity of a cause, isolated and inexplicable in the light of ignorance, then and only then may it be a source of enchantment.”
—Samuel Beckett (1906–1989)
“The knight slew the dragon,
The lady was gay,
They rode on together,
Away, away.”
—Unknown. This Is the Key (l. 38–41)