title: PostgreSQL sequence that ensure a unique ID
body: <p>Having a table with a columnn ID as primary key, and a column MyNumber that contains integers defined by the sequence myUniqueSequence. I would like to define myUniqueSequence in PostgreSQL that will return the next free and unique number for the column MyNumber.</p>
  
  <p>This means, the next time a new row is created programatically will start by number 1, if it's free it will use it for the column myNumber, if not, it tries with 2 and so on.</p>