In this chapter we learn about changing the definition of a database table after it has already been installed. This example expands on the previous chapters by adding a new field to the table to collect the date a user last visited a node. We learn more about hook_update_N() as well as db_add_field().
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This video covers a topic in Drupal 7 which may or may not be the version you're using. We're keeping this tutorial online as a courtesy to users of Drupal 7, but we consider it archived.