diff --git a/backend/lib/Interpreter/Interpreter.php b/backend/lib/Interpreter/Interpreter.php
index 70afa2b..b59d5d5 100644
--- a/backend/lib/Interpreter/Interpreter.php
+++ b/backend/lib/Interpreter/Interpreter.php
@@ -76,10 +76,8 @@ abstract class Interpreter implements InterpreterInterface {
 	 */
 	protected function getData($tuples = NULL, $groupBy = NULL) {
 		// prepare sql
-		$parameters = array(':id' => $this->channel->getId());
-
 		$sql['from']	= ' FROM data';
-		$sql['where']	= ' WHERE channel_id = :id' . self::buildDateTimeFilterSQL($this->from, $this->to);
+		$sql['where']	= ' WHERE channel_id = ?' . self::buildDateTimeFilterSQL($this->from, $this->to);
 		$sql['orderBy']	= ' ORDER BY timestamp ASC';
 
 		if ($groupBy && $sql['groupFields'] = self::buildGroupBySQL($groupBy)) {
@@ -94,10 +92,10 @@ abstract class Interpreter implements InterpreterInterface {
 		}
 
 		// get total row count for grouping
-		$rowCount = $this->conn->fetchColumn($sql['rowCount'], $parameters, 0);
+		$rowCount = $this->conn->fetchColumn($sql['rowCount'], array($this->channel->getId()), 0);
 
 		// query for data
-		$stmt = $this->conn->executeQuery('SELECT ' . $sql['fields'] . $sql['from'] . $sql['where'] . $sql['groupBy'] . $sql['orderBy'], $parameters);
+		$stmt = $this->conn->executeQuery('SELECT ' . $sql['fields'] . $sql['from'] . $sql['where'] . $sql['groupBy'] . $sql['orderBy'], array($this->channel->getId()));
 
 		// return iterators
 		if ($sql['groupBy'] || is_null($tuples) || $rowCount < $tuples) {
diff --git a/backend/lib/Interpreter/SensorInterpreter.php b/backend/lib/Interpreter/SensorInterpreter.php
index cf32d1c..00fb815 100644
--- a/backend/lib/Interpreter/SensorInterpreter.php
+++ b/backend/lib/Interpreter/SensorInterpreter.php
@@ -58,7 +58,8 @@ class SensorInterpreter extends Interpreter {
 	 * @return array (0 => timestamp, 1 => value)
 	 */
 	public function getMin() {
-		return array_map('floatval', $this->conn->fetchAssoc('SELECT value, timestamp FROM data WHERE channel_id = ' . (int) $this->channel->getId() . parent::buildDateTimeFilterSQL($this->from, $this->to) . ' ORDER BY value ASC LIMIT 1'));
+		$min = $this->conn->fetchAssoc('SELECT value, timestamp FROM data WHERE channel_id = ?' . parent::buildDateTimeFilterSQL($this->from, $this->to) . ' ORDER BY value ASC LIMIT 1',  array($this->channel->getId()));
+		return ($min) ? array_map('floatval', $min) : NULL;
 	}
 
 	/**
@@ -67,7 +68,8 @@ class SensorInterpreter extends Interpreter {
 	 * @return array (0 => timestamp, 1 => value)
 	 */
 	public function getMax() {
-		return array_map('floatval', $this->conn->fetchAssoc('SELECT value, timestamp FROM data WHERE channel_id = ' . (int) $this->channel->getId() . parent::buildDateTimeFilterSQL($this->from, $this->to) . ' ORDER BY value DESC LIMIT 1'));
+		$max = $this->conn->fetchAssoc('SELECT value, timestamp FROM data WHERE channel_id = ?' . parent::buildDateTimeFilterSQL($this->from, $this->to) . ' ORDER BY value DESC LIMIT 1', array($this->channel->getId()));
+		return ($max) ? array_map('floatval', $max) : NULL;
 	}
 
 	/**
@@ -76,7 +78,7 @@ class SensorInterpreter extends Interpreter {
 	 * @return float
 	 */
 	public function getAverage() {
-		return (float) $this->conn->fetchColumn('SELECT AVG(value) FROM data WHERE channel_id = ' . (int) $this->channel->getId() . parent::buildDateTimeFilterSQL($this->from, $this->to));
+		return (float) $this->conn->fetchColumn('SELECT AVG(value) FROM data WHERE channel_id = ?' . parent::buildDateTimeFilterSQL($this->from, $this->to), array($this->channel->getId()), 0);
 	}
 
 	/**