php - 如何使用 for 循环和 union all 按列进行分组

Question

您好，这是我的查询，我收到了 3 条记录，我只想要一条记录，我正在尝试按 studentid 进行分组

foreach ($gdFinaldata['gdskill'] as $skillId => $skillScore){

$filterQuery .= $union_all. "SELECT stu.student_pid

FROM tbl_students stu LEFT JOIN r_job_invitations jbi ON stu.student_email = jbi.email 

LEFT JOIN r_job_scores jsc  ON jsc.student_id = stu.student_pid 

WHERE job_id = ".$jobID."  AND skill_id = ".$skillId." AND gd_score >= ".$skillScore." ";

$union_all=" UNION ALL ";

} //for ends here

记录将如下所示:

SELECT stu.student_pid FROM tbl_students stu LEFT JOIN r_job_invitations jbi ON stu.student_email = jbi.email LEFT JOIN r_job_scores jsc ON jsc.student_id = stu.student_pid WHERE job_id = 88 AND skill_id = 3 AND gd_score >= 1   
UNION ALL
SELECT stu.student_pid FROM tbl_students stu LEFT JOIN r_job_invitations jbi ON stu.student_email = jbi.email LEFT JOIN r_job_scores jsc ON jsc.student_id = stu.student_pid WHERE job_id = 88  AND skill_id = 63 AND gd_score >= 2   
UNION ALL  
SELECT stu.student_pid FROM tbl_students stu LEFT JOIN r_job_invitations jbi ON stu.student_email = jbi.email LEFT JOIN r_job_scores jsc ON jsc.student_id = stu.student_pid WHERE job_id = 88  AND skill_id = 128 AND gd_score >= 3

在此之后，我想添加组，我该怎么做，我尝试这样做:

$groupby = "GROUP BY student_id";
$filterStudents = $conn->query($filterQuery.$groupby);

My issue is still returning 3 records i want to group by student

供引用:

Answer 1

有多种方法可以做到这一点。

• 使用union而不是union all，这将仅保留唯一的条目

  select ...
  union
  select ...
  union
  select ...
  

• 如果您想保留union all(为什么这样做？)，请在您的选择周围使用括号，并在其后附加group by

  select student_pid
  ( select student_pid, student_id from ...
  union all
  select student_pid, student_id from ...
  union all
  select student_pid, student_id from ... ) as t
  group by student_id
  

  你的 PHP 看起来会是这样的

  foreach (...) {
      $filterQuery .= $union_all. "SELECT stu.student_pid, jsc.student_id ..."
      // ...
  }
  
  $outerQuery = "select student_pid ("
  $groupby = ") as t GROUP BY student_id";
  $filterStudents = $conn->query($outerQuery . $filterQuery . $groupby);
  

• 除了group by之外，您还可以使用distinct，例如

  select distinct student_pid
  ( select student_pid from ...
  union all
  select student_pid from ...
  union all
  select student_pid from ... ) as t
  

• 将结果收集到数组中，然后使用 array_unique 尽管这是效率最低的，因为它获取所有条目而不是相关部分。