Multiple query statements in db_query?

db_query is Drupal's wrapper for mysql_query or mysqli_query (or whatever is appropriate for the selected database, such as pg_query for PostgreSQL database server).

Since the PHP functions mysql_query and mysqli_query can only execute one SQL statement, the db_query wrapper can also only execute one SQL statement.
("One SQL statement" means one statement regardless of line breaks..., It can optionally end with a semicolon, but cannot be more than one statement separated by semicolons).

If Drupal used mysqli_multi_query instead, then multiple SQL statements could be supported... but then this would require using PHP's "mysqli" instead of "mysql", and the Drupal wrapper would need to be able to return multiple result sets, thereby complicating the Drupal API. (Notice that while there is no pg_multi_query... it seems (according to PHP help) that pg_query DOES support multiple statements, though I have no PostgreSQL server to test with. )

This page http://drupal.org/node/119781 shows how to use multi_query, and you can mix it with the Drupal wrappers like this to make your own "my_multi_query", and use it similarly to db_query:

( Note two warnings, which are probably the reasons nothing like this is in "core":
1: Your $db_url in settings.php MUST be using "mysqli:", not "mysql:" or "pg:"
2: If returning large result sets, this function may not be practical since it stores all the results before returning. That is one major drawback of multi_query: You must deal with all the results before letting control get back to Drupal, since even a WatchDog call will cause a "sync" error, if you haven't finished retrieving all the result sets.
)

// use my_multi_query like this:

$query  = 'UPDATE {table} SET field=%s WHERE keyfield=%d; UPDATE {table} SET field=%s WHERE keyfield=%d;';
$args   = array( 'First New Value', 1,
                 'Second New Value', 2,
               );
$results = my_multi_query($query, $args);

foreach ($results as $result_rows) {
  foreach ($result_rows as $row) {
    // Do something with this row.
  };
};

function my_multi_query($query) {
global $active_db;

  $args = func_get_args();
  array_shift($args);
  $query = db_prefix_tables($query);
  if (isset($args[0]) and is_array($args[0])) { // 'All arguments in one array' syntax
    $args = $args[0];
  }
  _db_query_callback($args, TRUE);
  $query = preg_replace_callback(DB_QUERY_REGEXP, '_db_query_callback', $query);
  $results = array();

  if (_my_multi_query($query)) {

    // Process each result
    do {
    
      if ($result = mysqli_store_result($active_db)) {
    
        // Build an array of result rows
        while ($row = mysqli_fetch_array($result)) {
          $rows[] = $row;
        }

        // Add this array of rows to the array of results
        $results[] = $rows;
        mysqli_free_result($result);
      };
      
      // Get the next result
    } while (mysqli_next_result($active_db));

  };
  return $results;
}


function _my_multi_query($query, $debug = 0) {
  global $active_db, $queries, $user;

  if (variable_get('dev_query', 0)) {
    list($usec, $sec) = explode(' ', microtime());
    $timer = (float)$usec + (float)$sec;
    // If devel.module query logging is enabled, prepend a comment with the username and calling function
    // to the SQL string. This is useful when running mysql's SHOW PROCESSLIST to learn what exact
    // code is issueing the slow query.
    $bt = debug_backtrace();
    // t() may not be available yet so we don't wrap 'Anonymous'.
    $name = $user->uid ? $user->name : variable_get('anonymous', 'Anonymous');
    // str_replace() to prevent SQL injection via username or anonymous name.
    $name = str_replace(array('*', '/'), '', $name);
    $query = '/* '. $name .' : '. $bt[2]['function'] .' */ '. $query;
  }

  $result = mysqli_multi_query($active_db, $query);

  if (variable_get('dev_query', 0)) {
    $query = $bt[2]['function'] ."\n". $query;
    list($usec, $sec) = explode(' ', microtime());
    $stop = (float)$usec + (float)$sec;
    $diff = $stop - $timer;
    $queries[] = array($query, $diff);
  }

  if ($debug) {
    print '<p>query: '. $query .'<br />error:'. mysqli_error($active_db) .'</p>';
  }

  if (!mysqli_errno($active_db)) {
    return $result;
  }
  else {
    // Indicate to drupal_error_handler that this is a database error.
    ${DB_ERROR} = TRUE;
    trigger_error(check_plain(mysqli_error($active_db) ."\nquery: ". $query), E_USER_WARNING);
    return FALSE;
  }
}