多个选择查询与单个where子句？

I am creating a messaging system. In which there are two type of user admin and user.
Admin can send message to all user and user can send message to admin.
I am trying to fetch,

$this->db->select("user_from,count(user_from)as total");
$this->db->from("messages");
$this->db->group_by("user_from");
$query = $this->db->get();
return $query->result_array();

$this->db->select("*");
$this->db->from("messages");
$this->db->where("user_to","admin");
$query = $this->db->get();
return $query->result_array();

In single both the query work correctly.
I want to include both query in one.
I want to run multiple select query using single where clause which show the last message send by user to admin.
can anyone give me any idea?

table screenshot model code

Your Controller

$this->db->where("user_to","admin");
$this->db->group_by("user_from");
$query = $this->db->get('messages');
$data['result'] = $query->result_array(); // Your Result Array
$data['total'] = $query->num_rows(); // total number of records

// You can pass $data in any view file here

Your View

print_r($result); // result array
echo $total; // total number of records

You can use below query

$this->db->select("messages.*,count(messages.user_from) as total");
$this->db->from("messages");
$this->db->group_by("messages.user_from");
$this->db->order_by("messages.user_from","DESC");
$this->db->where("messages.user_to","admin");
$query = $this->db->get();
return $query->result_array();

$this->db->select("*,user_from,count(user_from)as total");
$this->db->from("messages");    
$this->db->group_by("user_from");
$this->db->where("user_to","admin");
$query = $this->db->get();
return $query->result_array();

To get the last message, use subquery like :

select message from messages where id = (select max(id) from messages where user_to = 'admin')

Test the query in MySql and if you get the result you want, you can go to codeigniter.

In Codeigniter:

 $this->db->select('message')->from('messages');
 $this->db->where('id = (SELECT max(id) FROM messages where user_to = 'admin')', NULL, FALSE);

Please correct the use of single quotes in above query as its demo only.

$this->db->select("*,user_from,count(user_from)as total");
$this->db->from("messages");    
$this->db->group_by("user_from");
$this->db->where("user_to","admin");
$query = $this->db->get();
return $query->result_array();

I have solved my problem.

Modal

function AdminMessages($user)
        {
            $sql = "select * from messages where id in (select max(id) from messages GROUP BY user_from ) AND user_to='".$user."' order by id desc";
            $query = $this->db->query($sql);
            //echo $this->db->last_query();
            return $query->result_array();
        }

        function UserMessages($user)
        {
            $sql = "select * from messages where id in (select max(id) from messages GROUP BY user_to ) AND user_to='".$user."' order by id desc";
            $query = $this->db->query($sql);
            //echo $this->db->last_query();
            return $query->result_array();
        }