链接数据库
-------
$this->load->database();//手动连接数据库
//连接多数据库
$DB1 = $this->load->database('group_one', TRUE);
$DB2 = $this->load->database('group_two', TRUE);
-----------------------------------------------------
查询
-------
//参数绑定形式
$sql = "SELECT * FROM some_table WHERE id = ? AND status = ? AND author = ?";
$this->db->query($sql, array(3, 'live', 'Rick'));
//多结果标准查询
$query = $this->db->query($sql); //自定义
$query = $this->db->get('tablename'); //便捷形式，相当于:SELECT * FROM tablename
$query = $this->db->get('tablename', 10, 20); // 相当于: SELECT * FROM tablename LIMIT 20, 10
$query->result() //对象形式
$query->result_array() //数组形式

$query->num_rows() //总条数
$query->num_fields() //字段数
//单结果标准查询
$row = $query->row(); //对象形式
$row = $query->row_array(); //数组形式

-----------------------------------------------------
插入
-------
$data = array(
                                'title' => $title,
                                'name' => $name
                                );
$this->db->insert('tablename', $data); //便捷插入
$this->db->insert_string('tablename', $data);  //便捷插入
$this->db->insert_id() //刚插入的id
$this->db->affected_rows() //影响的行数(update,insert)
-----------------------------------------------------
更新
-------
$data = array(
                                'name' => $name,
                                'email' => $email
                                );
$where = "id = 1";
$this->db->update('tablename', $data);
$this->db->update_string('tablename', $data, $where);
-----------------------------------------------------
删除
-------
$array = array(
                                'name' => $name,
                                'title' => $title
                                );
$this->db->delete('tablename', $array);
// Produces:
// "DELETE FROM tablename WHERE name = '$name' AND title = '$title'"
$this->db->truncate('tablename'); //清空表
// Produce: TRUNCATE tablename
-----------------------------------------------------
（where）
-------
$array = array(
                                'name' => $name,
                                'title' => $title
                                );
$this->db->where($array);
// Produces: "WHERE name = '$name' AND title = '$title'"
-----------------------------------------------------
$this->db->count_all('tablename'); //表中记录总行数
-----------------------------------------------------
$query->free_result() //释放资源