$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