title: SQL set values of one column equal to values of another column in the same table
body: <p>I have a table with two DATETIME columns.</p>
  
  <p>One of them is never NULL, but one of them is sometimes NULL.</p>
  
  <p>I need to write a query which will set all the NULL rows for column B equal to the values in column A.</p>
  
  <p>I have tried <a href="https://stackoverflow.com/questions/707371/sql-update-set-one-column-to-be-equal-to-a-value-in-a-related-table-referenced-b">this example</a> but the SQL in the selected answer does not execute because MySQL Workbench doesn't seem to like the FROM in the UPDATE.</p>