mysql - 左连接不能正常工作

Question

表 tbl_students

Student_pid     name                email
1               waheed               waheed@gmail.com
2               fareed               fareed@gmail.com

表 r_job_invitations

id_job   email
101       waheed@gmail.com
101       fareed@gmail.com
123       waheed@gmail.com
123       fareed@gmail.com

表 r_job_groups

student_id   job_id   group_id
1             101       1
2             101       2
1             123       1
2             123       2

从上面的 3 个表中，我试图让学生有条件。这是我的查询:

    $studentQuery = $conn->query("SELECT
      s.student_pid,jbi.test_status
       FROM `r_job_groups` jtg 
        LEFT JOIN tbl_students s ON jtg.student_id=s.student_pid 
        LEFT JOIN r_job_invitations jbi ON jbi.email=s.student_email 
         where jtg.group_id=".$group." and job_id=".$jobID );

从上面的查询中，对于值

$group = 1 and $jobID = 101

结果是这样的:

student_pid
1
1
2
2

实际结果应该是这样的:

student_pid
    1
    2

my issue is i am getting students doble time

根据查询，该结果应该给 2 个学生，但结果是 4 个学生，因为工作 ID 无法正常工作。
我该如何解决这个问题？

Answer 1

小心在你的选择中使用 vars 你可能会受到 sql 注入(inject)

无论如何你可以使用 distinct 来避免重复值

  $studentQuery = $conn->query("SELECT DISCINCT
  s.student_pid
   FROM `r_job_groups` jtg 
    LEFT JOIN tbl_students s ON jtg.student_id=s.student_pid 
    LEFT JOIN r_job_invitations jbi ON jbi.email=s.student_email 
     where jtg.group_id=".$group." and jtg job_id=".$jobID );

或者使用不同的动态表

  $studentQuery = $conn->query("SELECT 
  s.student_pid
   FROM `r_job_groups` jtg 
    LEFT JOIN ( select distinct student_pid 
           from tbl_students )  s ON jtg.student_id=s.student_pid 
    LEFT JOIN r_job_invitations jbi ON jbi.email=s.student_email 
     where jtg.group_id=".$group." and jtg.job_id=".$jobID );

并根据您的数据样本尝试更改 joi 表的顺序

$studentQuery = $conn->query("SELECT DISTINCT s.student_pid
      FROM  tbl_students s  
      LEFT `r_job_groups` jtg  s ON jtg.student_id=s.student_pid 
      LEFT JOIN r_job_invitations jbi ON jbi.email=s.student_email 
     where jtg.group_id=".$group." and job_id=".$jobID );