SET NAMES 'utf8';它相当于下面的三句指令:
SET character_set_client = utf8;
SET character_set_results = utf8;
SET character_set_connection = utf8;
SET character_set_client = utf8;
SET character_set_results = utf8;
SET character_set_connection = utf8;
这个只保证每次会话时的设置,一旦连接关闭后,设置后失效了