Does SQL have a condition like 'IS UNIQUE'? : <p>Wherever I was looking for, I've never heard about that any SQL implementations would have an <code>IS UNIQUE</code> "command". Did I miss something? If I'm right, what is the reason?</p>

<p>In other words, why I can't state in SQL that a certain command, such as <code>INSERT INTO</code> (<em>especially</em> that) can be done only if a value is or is not considered UNIQUE?</p>
