SIP Trace: Improve "group by Call-ID" query

    * be compatible with any SQL mode.  This fixes:

	SELECT DISTINCT callid FROM sip_trace WHERE status='' AND
	    direction='in' ORDER BY id DESC;

	ERROR 3065 (HY000): Expression #1 of ORDER BY clause is not in
	    SELECT list, references column 'opensips.sip_trace.id' which
	    is not in SELECT list; this is incompatible with DISTINCT

    * be consistent across different backends (e.g. don't sort the
	results differently with MySQL vs. Postgres)

Fixes #83

Author: liviuchircu

Diff for: web/tools/system/siptrace/template/tracer.main.php

@@ -134,11 +134,8 @@
 
 
 if ($_SESSION['grouped_results']) {
-	if ($config->db_driver == "mysql")
-		$sql = "SELECT DISTINCT callid FROM ".$table." WHERE status='' AND direction='in'".$sql_search." ORDER BY id DESC";
-	else if ($config->db_driver == "pgsql")
-		$sql = "SELECT DISTINCT ON (callid) callid FROM ".$table." WHERE status='' AND direction='in'".$sql_search." ORDER BY callid DESC";
-
+	$sql = "SELECT callid, MAX(id) AS mid FROM ".$table.
+				" WHERE status='' AND direction='in'".$sql_search." GROUP BY callid ORDER BY mid DESC";
 	$sql_cnt = "SELECT COUNT(DISTINCT(callid)) FROM ".$table." WHERE (1=1) ".$sql_search;
 } else {
 	$sql = "SELECT id FROM ".$table." WHERE (1=1) ".$sql_search." ORDER BY id DESC";