$this->db->select();       //选择查询的字段  
$this->db->select_max();  
$this->db->select_min();  
$this->db->select_avg();  
$this->db->select_sum();  
$this->db->from();         //选择表名  
$this->db->join();  
$this->db->get();          //得到查询结果  
$this->db->get_where();  
$this->db->where();  
$this->db->or_where();  
$this->db->where_in();  
$this->db->or_where_in();  
$this->db->where_not_in();  
$this->db->or_where_not_in();  
$this->db->like();  
$this->db->or_like();  
$this->db->not_like();  
$this->db->or_not_like();  
$this->db->group_by();  
$this->db->distinct();  
$this->db->having();  
$this->db->or_having();  
$this->db->order_by();  
$this->db->limit();  
$this->db->count_all_results();  

$this->db->insert();  

$this->db->set();  
$this->db->update();  

$this->db->delete();  

$this->db->empty_table();  
$this->db->truncate();  

$this->db->start_cache()  
$this->db->stop_cache()  
$this->db->flush_cache()  

result()        //返回对象数组  
result_array()  //返回二维数组  
row()           //返回一个对象  
row_array()     //返回一维数组  
num_rows()      //返回查询的行数  
num_fields()    //返回查询结果的字段数  
free_result()   //释放查询所占的资源内存  

$this->db->insert_id()       //获取刚刚插入的id  
$this->db->affected_rows()   //修改或插入影响的行数  
$this->db->count_all();      //统计记录的总条数 这一函数加入where条件无效  
$this->db->last_query();     //最后一条执行的sql语句  
  
//注意以下两个函数仅返回sql语句 不执行sql语句  
  
$data = array('name' => $name, 'email' => $email, 'url' => $url);  
$str = $this->db->insert_string('table_name', $data);  
  
$data = array('name' => $name, 'email' => $email, 'url' => $url);  
$where = "author_id = 1 AND status = 'active'";  
$str = $this->db->update_string('table_name', $data, $where); //返回正确格式的更新字符串