title: PHP - echo missing entries
body: <p>I have a problem.</p>
  
  <p>I have three tables <strong>class_tbl,subject_tbl, and *student_tbl</strong>. class_tbl has 3 columns *"class_id, subject_id, and student_id".* while subject_tbl has 4 columns *"subject_id,subject_name,subject_yr,subject_sem"* and lastly *student_tbl has 2 columns "student_id and student_name".*</p>
  
  <p>class_tbl has these rows:</p>
  
  <pre><code>**[class_id || subject_id || student_id]**
  
  || 1001   ||    2002    || 3003
  
  || 1002   ||    2003    || 3003
  
  || 1003   ||    2004    || 3003
  </code></pre>
  
  <p>subject_tbl has 5 rows:</p>
  
  <pre><code>**[subject_id ||  subject_name  || subject_yr || subject_sem]**
  
  || 2002     ||    Math        ||    2010    || 1st
  
  || 2003     ||   Science      ||    2010    || 1st
  
  || 2004     ||    History     ||    2010    || 1st
  
  || 2005     ||      PE        ||    2010    || 1st
  
  || 2006     || Social Studies ||    2010    || 1st
  </code></pre>
  
  <p>student_tbl has 1 row: depending on the number of students actually</p>
  
  <pre><code>**[student_id || student_name]**
  
  ||   3003   ||  John Parker
  </code></pre>
  
  <p>The goal here is that the query will echo the last 2 subjects which is PE and Social Studies so that I can insert them to class_tbl. The echo is telling me that I dont have 2 entries for the 2 subject during that semester and school year. The query will not echo the subjects which are already in the entries of class_tbl which are Math,Science, and History.
  How do I query and echo the missing subjects of that semester and year? I really cant come up with any idea. Help tnx!</p>
  
  <p>The code here displays all subjects of that school_yr and semester.</p>
  
  <pre><code>            if (isset ($db1)&amp;&amp;$db1!=""){
  
  
              }
  
              &lt;?php
  
                  $student_id = $_POST['student_id'];
                  $sem = $_POST['sem'];
                  $yr = $_POST['yr'];
  
                  $list = mysql_query("SELECT * 
                                      FROM
                                      class_tbl 
                                      LEFT JOIN
                                      subject_tbl
                                      ON
                                      class_tbl.subject_id = subject_tbl.subject ID
                                      WHERE
                                      student_id = '$student_id'
                                      OR
                                      subject_sem = '$sem' 
                                      OR
                                      subject_yr = '$yr' 
                                      ORDER BY 
                                      subject_id asc");
  
                  while($row_list=mysql_fetch_assoc($list))
                  {
                  echo $row_list['subject_id'];
  
                  if($row_list['subject_id']==$db1){echo "selected"; }
  
                  echo $row_list['subject_name'];
  
                  }
  </code></pre>
  
  <p>The example output would be:</p>
  
  <p>You have 2 missing subjects "PE and Social Studies."                </p>