Drupal 7 SQL FAQ: How do I perform a SQL DELETE query in Drupal 7?
The short answer is that you use the Drupal 7 db_delete function, like this:
db_delete('projects') ->condition('id', 10) ->condition('user_id', 5) ->execute();
This example assumes that your database fields are named 'id' and 'user_id', and the values of 10 and 5 are the values in your database table you want to delete. If you just want to delete by id, your query is simpler, like this:
db_delete('projects') ->condition('id', 10) ->execute();
Or, if you prefer seeing PHP variables, my current code looks like this:
db_delete('sleet_projects') ->condition('id', $project_id_to_delete) ->condition('user_id', get_user_id()) ->execute();
For more information on this SQL DELETE function, see the Drupal 7 db_delete documentation page.
I'll be glad to share more code related to the SQL DELETE process in a Drupal form/module; if you're interested in seeing my Drupal form delete function and submit handler, just leave a note here and I'll be glad to share all of that.