PHP-回显缺少的条目

我有个问题

SELECT  a.*
FROM    subject_tbl a
        LEFT JOIN class_tbl b
            ON  a.subject_ID = b.subject_ID AND
                a.subject_yr = 2010 AND
                a.subject_sem = '1st' AND
                b.student_ID = 3003
WHERE   b.subject_ID IS NULL

我有三张桌子，分别是班级、学科和*学生。班级tbl有3列*“班级id、科目id和学生id”。*而科目tbl有4列*“科目id、科目名称、科目年份、科目结构”*最后*学生tbl有2列“学生id和学生名称”*

SELECT  a.*
FROM    subject_tbl a
        LEFT JOIN class_tbl b
            ON  a.subject_ID = b.subject_ID AND
                a.subject_yr = 2010 AND
                a.subject_sem = '1st' AND
                b.student_ID = 3003
WHERE   b.subject_ID IS NULL

类tbl有以下行：

**[class_id || subject_id || student_id]**

|| 1001   ||    2002    || 3003

|| 1002   ||    2003    || 3003

|| 1003   ||    2004    || 3003

SELECT  a.*
FROM    subject_tbl a
        LEFT JOIN class_tbl b
            ON  a.subject_ID = b.subject_ID AND
                a.subject_yr = 2010 AND
                a.subject_sem = '1st' AND
                b.student_ID = 3003
WHERE   b.subject_ID IS NULL

主题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

SELECT  a.*
FROM    subject_tbl a
        LEFT JOIN class_tbl b
            ON  a.subject_ID = b.subject_ID AND
                a.subject_yr = 2010 AND
                a.subject_sem = '1st' AND
                b.student_ID = 3003
WHERE   b.subject_ID IS NULL

学生tbl有一行：取决于实际学生人数

**[student_id || student_name]**

||   3003   ||  John Parker

SELECT  a.*
FROM    subject_tbl a
        LEFT JOIN class_tbl b
            ON  a.subject_ID = b.subject_ID AND
                a.subject_yr = 2010 AND
                a.subject_sem = '1st' AND
                b.student_ID = 3003
WHERE   b.subject_ID IS NULL

这里的目标是，查询将回应最后两个主题，即体育和社会研究，以便我可以将它们插入课堂。echo告诉我，在那个学期和学年，我没有2个科目的条目。该查询不会回显已在数学、科学和历史类tbl条目中的科目。 我如何质疑和回应那学期和那一年缺少的科目？我真的想不出什么主意。帮助tnx

SELECT  a.*
FROM    subject_tbl a
        LEFT JOIN class_tbl b
            ON  a.subject_ID = b.subject_ID AND
                a.subject_yr = 2010 AND
                a.subject_sem = '1st' AND
                b.student_ID = 3003
WHERE   b.subject_ID IS NULL

这里的代码显示了该学校一年和一学期的所有科目

            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'];

                }

SELECT  a.*
FROM    subject_tbl a
        LEFT JOIN class_tbl b
            ON  a.subject_ID = b.subject_ID AND
                a.subject_yr = 2010 AND
                a.subject_sem = '1st' AND
                b.student_ID = 3003
WHERE   b.subject_ID IS NULL

if（isset（$db1）&&$db1！=“”）{
}



输出

SELECT  a.*
FROM    subject_tbl a
        LEFT JOIN class_tbl b
            ON  a.subject_ID = b.subject_ID AND
                a.subject_yr = 2010 AND
                a.subject_sem = '1st' AND
                b.student_ID = 3003
WHERE   b.subject_ID IS NULL

╔════════════╦════════════════╦════════════╦═════════════╗
║ SUBJECT_ID ║  SUBJECT_NAME  ║ SUBJECT_YR ║ SUBJECT_SEM ║
╠════════════╬════════════════╬════════════╬═════════════╣
║       2005 ║ PE             ║       2010 ║ 1st         ║
║       2006 ║ Social Studies ║       2010 ║ 1st         ║
╚════════════╩════════════════╩════════════╩═════════════╝




输出

SELECT  a.*
FROM    subject_tbl a
        LEFT JOIN class_tbl b
            ON  a.subject_ID = b.subject_ID AND
                a.subject_yr = 2010 AND
                a.subject_sem = '1st' AND
                b.student_ID = 3003
WHERE   b.subject_ID IS NULL

╔════════════╦════════════════╦════════════╦═════════════╗
║ SUBJECT_ID ║  SUBJECT_NAME  ║ SUBJECT_YR ║ SUBJECT_SEM ║
╠════════════╬════════════════╬════════════╬═════════════╣
║       2005 ║ PE             ║       2010 ║ 1st         ║
║       2006 ║ Social Studies ║       2010 ║ 1st         ║
╚════════════╩════════════════╩════════════╩═════════════╝

如果您使用，请将其结构设计得更人性化一点（例如，将内容加粗或斜体），然后帮助您轻松地将其结构设计得更人性化一点（例如，将内容加粗或斜体），如果您使用，那么帮助您轻松地使用许多tnx。。。最后一个问题。请问是否可以，SELECT和class_tbl“b”中的“a.*”代表什么？它们应该是桌子吗？对不起，
a和b分别被称为表
subject\u tbl
和
class\u tbl
的别名
a.*
表示从表
subject\u tbl
中选择所有列。不客气
：）
别名在表名较长时很有用。嘿，伙计，虽然有些科目属于不同的学期和学年，但为什么查询会显示重复项呢？即使我指定了学期和学年。例如，历史分为2010学年的第二学期和2011学年的第一学期。回声显示两个同名条目。添加不同的，例如
选择不同的a.*FROM…
Many tnx to u…，怎么样。。。最后一个问题。请问是否可以，SELECT和class_tbl“b”中的“a.*”代表什么？它们应该是桌子吗？对不起，
a和b分别被称为表
subject\u tbl
和
class\u tbl
的别名
a.*
表示从表
subject\u tbl
中选择所有列。不客气
：）
别名在表名较长时很有用。嘿，伙计，虽然有些科目属于不同的学期和学年，但为什么查询会显示重复项呢？即使我指定了学期和学年。例如，历史分为2010学年的第二学期和2011学年的第一学期。回显显示两个相同名称的条目。添加不同名称的条目如何，例如
选择不同的a.*FROM…