Question

I am stuck with a mysql query. I have three tables:

Table Name : Teacher

teacherid  teachername 
 1001           ABC
 1002           XYZ

Table Name : batch

batchid   batchname  batch_type     class   batchinstructor     seat
   1        Solar        1            Five     1001               40
   2        Earth        2            Six      1002               40

Table Name: student

    studentid studentname batchid
     1          Bon Jovi     1
     2          Shane Warne  2
     3          John Denver  1
     4          Steve Wuagh  2
     5          Perterson    2

What I want to achieve

 batchname class  teachername seat  students_enrolled  seat_available
   Solar   Five    ABC          40           2              38
   Earth   six     XYZ          40           3              37

I am using codeigniter and have managed to achieve all the things I have mentioned above accept for the column students_enrolled, seat_available

Here’s the code what I have tried.

      function batch_list($perPage,$uri) { 
    $this->db->select('*');
    $this->db->from('batch');
    $this->db->join('teacher', 'batch.batchinstructor = teacher.teacherid');
    $this->db->order_by('batchid','DESC');
    $getData = $this->db->get('', $perPage, $uri);
    if($getData->num_rows() > 0)
    return $getData->result_array();
        else
        return null;
    }

Would you please kindly help me with the mysql query. please note that I am using Codeigniter.

thanks in advance

Answer 1

In pure SQL, the query you want is:

SELECT
  b.batchname, b.class, t.teachername, b.seat,
  COUNT(s.studentid) AS students_enrolled,
  b.seat - COUNT(s.studentid) AS seat_available
FROM batch b
LEFT JOIN Teacher t ON t.teacherid = b.batchinstructor
LEFT JOIN students s ON s.batchid = b.batchid
GROUP BY s.batchid
ORDER BY b.batchid DESC

All you need to do is translate this CodeIgniter – as suggested by @Broncha, you can do:

$result = $this->db->query($sql);