This article explains a practical implementation of a technique outlined in the article "Sharing Drupal tables between databases using MySQL5 Views".
You have multiple (multisite) Drupal sites and you would like to manage the content for all of these sites through a single interface. Depending on the nature of a given piece of content, you may want the content published on one, several or all of your subsites, but you do not want to have to create copies of the same content for each site.
Taxonomy plus MySQL5 views. (NOTE: this solution will not work with versions of MySQL prior to 5.)
Assumming you have your subsites properly set up and running, the first step is to create a special vocabulary which you will use to target content.
Go to [your site's baseurl]/admin/taxonomy/add/vocabulary and create a vocabulary. We'll call it simply "sites".
Next, go back to your taxonomy page (/admin/taxonomy) and select "edit vocabulary" for the "sites" vocabulary.
Add a name for each of the subsites you would like to manage. For our example, we'll have two subsites, "foo" and "bar", and one master site, "master".
Now add at least three pieces of test content. Target one piece of content for each of foo, bar and both.
Next, we're going to create a node view for each of our subsites that we'll use to replace the actual node table.
The SQL is as follows:
CREATE VIEW [subsite, eg. "foo"]_node AS SELECT n.* FROM node n, term_data td, term_node tn, vocabulary v WHERE v.name = '[vocabulary name, eg. "sites"]' AND td.vid = v.vid AND td.name = '[subsite vocab term, eg. "foo"]' AND td.tid = tn.tid AND n.nid = tn.nid ;
Because the terms that serve as our subsite labels may very well exist within other vocabularies, we also need to join on the vocabulary table to ensure our solution works reliabley.
Finally, we need to have our subsites use the views we have created instead of our master nodes table, which only the "master" site will have access to directly.
In your drupal's sites directory, you should have directories that correspond to each of your drupal sites (both master and subsites). Edit the settings.php file for each of your subsites, and use the db_prefix variable to point the site to your view. So sites/foo.example.com/settings.php would contain the following:
$db_prefix = array( 'node' => 'foo_', );
At this point, you'll want to disable creation of content from within each of your subsites. You can do this in the from the admin/access page. If you attempt to create content from within the subsites, you'll likely get a 'duplicate key' error.
I hope that explanation is clear. These articles are written rather hastily. If you questions or suggestions regarding this solution, please leave a comment.