Add Expression class

Author: technicalguru

src/TgDatabase/Expression.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace TgDatabase;
+
+/**
+ * An object-oriented representation of a select clause part in a Query. 
+ * Built-in select types are provided by the Projections factory class. This interface 
+ * might be implemented by application classes that define custom select component. 
+ */
+interface Expression {
+
+	/**
+	  * Render the SQL fragment.
+	  * @param Query $localQuery   - local query object (e.g. subquery)
+	  * @param Query $overallQuery - overall query object
+	  * @return string - the SQL fragment representing this component.
+	  */
+	public function toSqlString($localQuery, $overallQuery);
+
+}