我有个问题。
我有三个表class_tbl、subject_tbl 和 *student_tbl。class_tbl 有 3 列 *"class_id、subject_id 和 student_id"。* 而 subject_tbl 有 4 列 *"subject_id,subject_name,subject_yr,subject_sem"*，最后 *student_tbl 有 2 列 "student_id 和 student_name"。*
class_tbl 有这些行：
**[class_id || subject_id || student_id]**
|| 1001 || 2002 || 3003
|| 1002 || 2003 || 3003
|| 1003 || 2004 || 3003
subject_tbl 有 5 行：
**[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
student_tbl 有 1 行：取决于实际的学生人数
**[student_id || student_name]**
|| 3003 || John Parker
这里的目标是查询将回显最后两个主题，即体育和社会研究，以便我可以将它们插入到 class_tbl。回声告诉我，在那个学期和学年，我没有 2 个科目的 2 个条目。该查询不会回显已经在 class_tbl 条目中的主题，即数学、科学和历史。如何查询和回显该学期和当年的缺失科目？我真的想不出任何想法。帮助tnx！
此处的代码显示了该学年和学期的所有科目。
if (isset ($db1)&&$db1!=""){
}
<?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'];
}
示例输出将是：
您有 2 个缺失的科目“体育和社会研究”。