diff --git a/framework/yii/db/ActiveQuery.php b/framework/yii/db/ActiveQuery.php
index 52dbcdb..e93e9be 100644
--- a/framework/yii/db/ActiveQuery.php
+++ b/framework/yii/db/ActiveQuery.php
@@ -123,6 +123,9 @@ class ActiveQuery extends Query implements ActiveQueryInterface
 		}
 
 		if ($this->sql === null) {
+			$select = $this->select;
+			$from = $this->from;
+
 			if ($this->from === null) {
 				$tableName = $modelClass::tableName();
 				if ($this->select === null && !empty($this->join)) {
@@ -130,8 +133,14 @@ class ActiveQuery extends Query implements ActiveQueryInterface
 				}
 				$this->from = [$tableName];
 			}
-			list ($this->sql, $this->params) = $db->getQueryBuilder()->build($this);
+			list ($sql, $params) = $db->getQueryBuilder()->build($this);
+
+			$this->select = $select;
+			$this->from = $from;
+		} else {
+			$sql = $this->sql;
+			$params = $this->params;
 		}
-		return $db->createCommand($this->sql, $this->params);
+		return $db->createCommand($sql, $params);
 	}
 }
diff --git a/framework/yii/db/Query.php b/framework/yii/db/Query.php
index 301b2e1..0d81887 100644
--- a/framework/yii/db/Query.php
+++ b/framework/yii/db/Query.php
@@ -190,8 +190,7 @@ class Query extends Component implements QueryInterface
 	 */
 	public function count($q = '*', $db = null)
 	{
-		$this->select = ["COUNT($q)"];
-		return $this->createCommand($db)->queryScalar();
+		return $this->queryScalar("COUNT($q)", $db);
 	}
 
 	/**
@@ -204,8 +203,7 @@ class Query extends Component implements QueryInterface
 	 */
 	public function sum($q, $db = null)
 	{
-		$this->select = ["SUM($q)"];
-		return $this->createCommand($db)->queryScalar();
+		return $this->queryScalar("SUM($q)", $db);
 	}
 
 	/**
@@ -218,8 +216,7 @@ class Query extends Component implements QueryInterface
 	 */
 	public function average($q, $db = null)
 	{
-		$this->select = ["AVG($q)"];
-		return $this->createCommand($db)->queryScalar();
+		return $this->queryScalar("AVG($q)", $db);
 	}
 
 	/**
@@ -232,8 +229,7 @@ class Query extends Component implements QueryInterface
 	 */
 	public function min($q, $db = null)
 	{
-		$this->select = ["MIN($q)"];
-		return $this->createCommand($db)->queryScalar();
+		return $this->queryScalar("MIN($q)", $db);
 	}
 
 	/**
@@ -246,8 +242,7 @@ class Query extends Component implements QueryInterface
 	 */
 	public function max($q, $db = null)
 	{
-		$this->select = ["MAX($q)"];
-		return $this->createCommand($db)->queryScalar();
+		return $this->queryScalar("MAX($q)", $db);
 	}
 
 	/**
@@ -258,8 +253,23 @@ class Query extends Component implements QueryInterface
 	 */
 	public function exists($db = null)
 	{
-		$this->select = [new Expression('1')];
-		return $this->scalar($db) !== false;
+		return $this->queryScalar(new Expression('1'), $db) !== false;
+	}
+
+	/**
+	 * Queries a scalar value by setting [[select]] first.
+	 * Restores the value of select to make this query reusable.
+	 * @param string|Expression $selectExpression
+	 * @param Connection $db
+	 * @return bool|string
+	 */
+	private function queryScalar($selectExpression, $db)
+	{
+		$select = $this->select;
+		$this->select = [$selectExpression];
+		$command = $this->createCommand($db);
+		$this->select = $select;
+		return $command->queryScalar();
 	}
 
 	/**