title: when do we use unique index and unique key?
body: <p>After some research over the net about <b>Unique key</b> and <b>unique index</b>. I have understand that both of them validate the column however unique index is better for performance.<br/>
  what is better, adding a unique key or unique index expecialy if I got a table with 500+ row, (I already used primary key for other columns).<br/>
  - If unique index is better because of performance, then when do we use unique key? as both of them accept null valuable(by the way can we make them accept only not null valuable ? like'add unique key not null' ?)<br/>-if unique key is better , what if after a year the table is 2000 rows , I am not sure if these table will have alot of records in the future.<br/></p>
  
  <p>edit:<br/>
  I was trying to use unique key on a duplicate column, even with novalidate it throws me an error. I had to creat an index on the duplicate column. </p>