The TIMESTAMP column type provides a type that you can use to 
automatically mark INSERT or UPDATE operations with the current date
and time. If you have multiple TIMESTAMP columns, only the first 
one is updated automatically. 

Automatic updating of the first TIMESTAMP column occurs under any 
of the following conditions: 

1. The column is not specified explicitly in an INSERT or 
   LOAD DATA INFILE statement. 
2. The column is not specified explicitly in an UPDATE 
   statement and some other column changes value. (Note 
   that an UPDATE that sets a column to the value it 
   already has will not cause the TIMESTAMP column to be 
   updated, because if you set a column to its current 
   value, MySQL ignores the update for efficiency.) 
3. You explicitly set the TIMESTAMP column to NULL.