Index: includes/database/database.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/database/database.inc,v
retrieving revision 1.31
diff -u -p -r1.31 database.inc
--- includes/database/database.inc	2 Dec 2008 19:45:01 -0000	1.31
+++ includes/database/database.inc	3 Dec 2008 04:28:43 -0000
@@ -1687,7 +1687,8 @@ function db_delete($table, Array $option
  * Returns a new SelectQuery object for the active database.
  *
  * @param $table
- *   The base table for this query.
+ *   The base table for this query.  May be a string or another SelectQuery
+ *   object.  If a query object is passed, it will be used as a subselect.
  * @param $alias
  *   The alias for the base table of this query.
  * @param $options
Index: includes/database/select.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/database/select.inc,v
retrieving revision 1.8
diff -u -p -r1.8 select.inc
--- includes/database/select.inc	8 Nov 2008 22:52:24 -0000	1.8
+++ includes/database/select.inc	3 Dec 2008 04:28:43 -0000
@@ -33,13 +33,16 @@ class SelectQuery extends Query implemen
    *
    * array(
    *   'type' => $join_type (one of INNER, LEFT OUTER, RIGHT OUTER),
-   *   'table' => $name_of_table,
+   *   'table' => $table,
    *   'alias' => $alias_of_the_table,
    *   'condition' => $condition_clause_on_which_to_join,
    *   'arguments' => $array_of_arguments_for_placeholders_in_the condition.
    *   'all_fields' => TRUE to SELECT $alias.*, FALSE or NULL otherwise.
    * )
    *
+   * If $table is a string, it is taken as the name of a table.  If it is
+   * a SelectQuery object, it is taken as a subquery.
+   *
    * @var array
    */
   protected $tables = array();
@@ -262,7 +265,8 @@ class SelectQuery extends Query implemen
   /**
    * Compiles and returns an associative array of the arguments for this prepared statement.
    *
-   * @return array
+   * @return
+   *   An associative array of all placeholder arguments for this query.
    */
   public function getArguments() {
     $this->where->compile($this->connection);
@@ -272,6 +276,10 @@ class SelectQuery extends Query implemen
       if ($table['arguments']) {
         $args += $table['arguments'];
       }
+      // If this table is a subquery, grab its arguments recursively.
+      if ($table['table'] instanceof SelectQuery) {
+        $args += $table['table']->getArguments();
+      }
     }
     foreach ($this->expressions as $expression) {
       if ($expression['arguments']) {
@@ -285,19 +293,7 @@ class SelectQuery extends Query implemen
   public function execute() {
     drupal_alter('query', $this);
 
-    $this->where->compile($this->connection);
-    $this->having->compile($this->connection);
-    $args = $this->where->arguments() + $this->having->arguments();
-    foreach ($this->tables as $table) {
-      if ($table['arguments']) {
-        $args += $table['arguments'];
-      }
-    }
-    foreach ($this->expressions as $expression) {
-      if ($expression['arguments']) {
-        $args += $expression['arguments'];
-      }
-    }
+    $args = $this->getArguments();
 
     if (!empty($this->range)) {
       return $this->connection->queryRange((string)$this, $args, $this->range['start'], $this->range['length'], $this->queryOptions);
@@ -548,8 +544,11 @@ class SelectQuery extends Query implemen
    * In some cases, that may include dipping into the Schema API to find the necessary
    * fields on which to join.
    *
+   * @param $type
+   *   The type of join.  Typically one one of INNER, LEFT OUTER, and RIGHT OUTER.
    * @param $table
-   *   The table against which to join.
+   *   The table against which to join.  May be a string or another SelectQuery
+   *   object.  If a query object is passed, it will be used as a subselect.
    * @param $alias
    *   The alias for the table.  In most cases this should be the first letter
    *   of the table, or the first letter of each "word" in the table.  If omitted,
@@ -568,7 +567,12 @@ class SelectQuery extends Query implemen
   public function addJoin($type, $table, $alias = NULL, $condition = NULL, $arguments = array()) {
 
     if (empty($alias)) {
-      $alias = $table;
+      if ($table instanceof SelectQuery) {
+        $alias = 'subquery';
+      }
+      else {
+        $alias = $table;
+      }
     }
 
     $alias_candidate = $alias;
@@ -699,9 +703,19 @@ class SelectQuery extends Query implemen
       if (isset($table['join type'])) {
         $query .= $table['join type'] . ' JOIN ';
       }
+
+      // If the table is a subquery, compile it and integrate it into this query.
+      if ($table['table'] instanceof SelectQuery) {
+        $table_string = '(' . (string)$table['table'] .')';
+      }
+      else {
+        $table_string = '{' . $this->connection->escapeTable($table['table']) . '}';
+      }
+
       // Don't use the AS keyword for table aliases, as some
       // databases don't support it (e.g., Oracle).
-      $query .= '{' . $this->connection->escapeTable($table['table']) . '} ' . $table['alias'];
+      $query .=  $table_string . ' ' . $table['alias'];
+
       if (!empty($table['condition'])) {
         $query .= ' ON ' . $table['condition'];
       }
Index: modules/simpletest/tests/database_test.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/simpletest/tests/database_test.module,v
retrieving revision 1.3
diff -u -p -r1.3 database_test.module
--- modules/simpletest/tests/database_test.module	8 Nov 2008 04:45:36 -0000	1.3
+++ modules/simpletest/tests/database_test.module	3 Dec 2008 04:28:43 -0000
@@ -59,3 +59,4 @@ function database_test_db_query_temporar
   print db_query('SELECT COUNT(*) FROM temporary')->fetchField();
   exit;
 }
+
Index: modules/simpletest/tests/database_test.test
===================================================================
RCS file: /cvs/drupal/drupal/modules/simpletest/tests/database_test.test,v
retrieving revision 1.23
diff -u -p -r1.23 database_test.test
--- modules/simpletest/tests/database_test.test	26 Nov 2008 13:48:49 -0000	1.23
+++ modules/simpletest/tests/database_test.test	3 Dec 2008 04:28:43 -0000
@@ -1149,6 +1149,82 @@ class DatabaseSelectTestCase extends Dat
 }
 
 /**
+ * Test case for subselects in a dynamic SELECT query.
+ */
+class DatabaseSelectSubqueryTestCase extends DatabaseTestCase {
+
+  function getInfo() {
+    return array(
+      'name' => t('Select tests, subqueries'),
+      'description' => t('Test the Select query builder.'),
+      'group' => t('Database'),
+    );
+  }
+
+  /**
+   * Test that we can use a subquery in a FROM clause.
+   */
+  function testFromSubquerySelect() {
+    try {
+      // Create a subquery, which is just a normal query object.
+      $subquery = db_select('test_task', 'tt');
+      $subquery->addField('tt', 'pid', 'pid');
+      $subquery->addField('tt', 'task', 'task');
+      $subquery->condition('priority', 1);
+
+      // Create another query that joins against the virtual table resulting
+      // from the subquery.
+      $select = db_select($subquery, 'tt2');
+      $select->join('test', 't', 't.id=tt2.pid');
+      $select->addField('t', 'name');
+
+      $select->condition('task', 'code');
+
+      // The resulting query should be equivalent to:
+      // SELECT t.name
+      // FROM (SELECT tt.pid AS pid, tt.task AS task FROM test_task tt WHERE priority=1) tt
+      //   INNER JOIN test t ON t.id=tt.pid
+      // WHERE tt.task = 'code'
+      $people = $select->execute()->fetchCol();
+
+      $this->assertEqual(count($people), 1, t('Returned the correct number of rows.'));
+    }
+    catch(Exception $e) {
+      $this->assertTrue(FALSE, $e->getMessage());
+    }
+  }
+
+  /**
+   * Test that we can use a subquery in a JOIN clause.
+   */
+  function testJoinSubquerySelect() {
+    try {
+      // Create a subquery, which is just a normal query object.
+      $subquery = db_select('test_task', 'tt');
+      $subquery->addField('tt', 'pid', 'pid');
+      $subquery->condition('priority', 1);
+
+      // Create another query that joins against the virtual table resulting
+      // from the subquery.
+      $select = db_select('test', 't');
+      $select->join($subquery, 'tt', 't.id=tt.pid');
+      $select->addField('t', 'name');
+
+      // The resulting query should be equivalent to:
+      // SELECT t.name
+      // FROM test t
+      //   INNER JOIN (SELECT tt.pid AS pid FROM test_task tt WHERE priority=1) tt ON t.id=tt.pid
+      $people = $select->execute()->fetchCol();
+
+      $this->assertEqual(count($people), 2, t('Returned the correct number of rows.'));
+    }
+    catch(Exception $e) {
+      $this->assertTrue(FALSE, $e->getMessage());
+    }
+  }
+}
+
+/**
  * Test select with order by clauses.
  */
 class DatabaseSelectOrderedTestCase extends DatabaseTestCase {