title: Adding column with datetime datatype using alter table statement but with a different value in each row
body: <p>I have a relation named Tweets with some columns. Now, I want to add a column named Timestamp with data type DATETIME but the problem is if I set a default value using now() all the rows get the same value. I want every row to have a different value. I cannot use rand() function along with now() as Mysql shall throw an error. So, how can I achieve this? Requesting assistance.</p>
  
  <p>table creation code:</p>
  
  <pre><code>create table Tweets(
  tweet_id int primary key,
  tweet_data varchar(160) not null,
  posted_by int not null,
  type int not null,
  belongs_to_nest int,
  constraint posted_by_foreign_key foreign key(posted_by) references Users(user_id),
  constraint type_foreign_key foreign key(type) references Types(type_id),
  constraint belongs_to_nest_foreign_key foreign key(belongs_to_nest) references Nests(nest_id)
  ); 
  </code></pre>
  
  <p>I am using this query as of now:</p>
  
  <pre><code>alter table Tweets add column Timestamp datetime not null unique default (now());
  </code></pre>
  
  <p>How should I modify this query so as to get a unique value in each row. From unique I mean is seconds must be different in each row although date, hours and minutes may be same.</p>