php - Mysql查询: Number of candidates enrolled and seat available

Question

我被 mysql 查询困住了。我有三张 table :

表名称:教师

teacherid  teachername 
 1001           ABC
 1002           XYZ

表名称:批处理

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

表名称:学生

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

我想要实现的目标

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

我正在使用 codeigniter，并成功实现了我上面提到的所有内容，接受 students_enrolled、seat_available 列

这是我尝试过的代码。

      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;
    }

请您帮我解决一下 mysql 查询好吗？请注意，我使用的是 Codeigniter。

提前致谢

Answer 1

在纯SQL中，你想要的查询是:

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

您需要做的就是翻译此 CodeIgniter - 按照 @Broncha 的建议，您可以执行以下操作:

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