title: Db2 change a column to row update timestamp
body: <p>In my table I've a column MODIFIED with type TIMESTAMP.
  I want to update this column automatically when the row is updated</p>
  
  <p>I can use this for a new column </p>
  
  <pre><code>alter table my_table add column last_updated timestamp not null
   generated by default for each row on update as row change timestamp 
  </code></pre>
  
  <p>But I can't use this to edit the existing column to do the updating automatically</p>
  
  <pre><code>alter table my_table alter column modified set data type timestamp 
   not null generated by default for each row on update as row change timestamp
  </code></pre>
  
  <p>Am I using the alter correctly? </p>
  
  <p>the error message is </p>
  
  <pre><code>Error: DB2 SQL Error: SQLCODE=-104, SQLSTATE=42601, SQLERRMC=alter table my_table alter column modified;BEGIN-OF-STATEMENT;&lt;values&gt;, DRIVER=3.50.152
  SQLState:  42601
  ErrorCode: -104
  </code></pre>