Add group by support in sql criteria.

model/SqlDbDriver.php

@@ -159,6 +159,30 @@ abstract class SqlDbDriver extends DbDriver
     }
 
     /**
+     * @param mixed $group_by
+     * @return string
+     * @throws ErrorException
+     */
+    public function groupByExpr($group_by)
+    {
+        if (empty($group_by)) {
+            return $group_by;
+        }
+
+        if (!is_array($group_by)) {
+            $group_by = array($group_by);
+        }
+        foreach ($group_by as &$term) {
+            if ($term instanceof DbExpr) {
+                $term = (string) $term;
+            } else {
+                $term = $this->quoteIdentifier($term);
+            }
+        }
+        return implode(',', $group_by);
+    }
+
+    /**
      * @param $select array
      * @param $distinct string|bool
      * @return string