Detailed explanation of the effect of secondary linkage menu implemented by php+mysql

The example in this article describes the secondary linkage menu effect achieved by php+mysql. Share it with everyone for your reference, the details are as follows:

<!--php+mysql二级联动-->


<meta http-equiv="Content-Type" c>
<title>生成学院专业级联下拉菜单测试 </title>



<!--************ JavaScript处理 college-onChange *************-->
<script language="JavaScript">
 var majorCount; // 存储专业记录条数
 // form_majors[] 储存专业 major数据，如 {(1,1,电子商务 ),(4,1,计算机科学 ),(3,2,古典文学 )}
 form_majors = new Array();
 <&#63;php
   $num2 = count($majors); // $num2 获取专业表中记录的个数
 &#63;>
   majorCount = <&#63;php echo $num2;&#63;>;
 <&#63;
   for($j=0;$j<$num2;$j++) // 从 0开始取出上面 majors[]中存储的专业数据填充数组
 {
 &#63;>
   form_majors[<&#63;echo $j;&#63;>] = new Array("<&#63;echo $majors[$j]['major_id'];&#63;>","<&#63;echo $majors[$j]['college_id'];&#63;>","<&#63;echo $majors[$j]['name'];&#63;>");
 <&#63;php
 }
 &#63;>
 function changeCollege(college_id)
 {
   document.stu_add_form.major.length = 0;
   var id=id;
   var j;
   document.stu_add_form.major.options[0] = new Option('==选择专业 ==',''); // label的 value为空 ' '
   for (j=0;j < majorCount; j++) // 从 0开始判断
   {
    if (form_majors[j][1] == college_id) // if college_id等于选择的学院的 id
    {
       document.stu_add_form.major.options[document.stu_add_form.major.length] = new Option(form_majors[j][2], form_majors[j][0]);
    }
   }
 }
</script>
<!--********************页面表单 *************************-->

选择： ==请选择学院 == ==选择专业 ==

sql statement:

--
-- 表的结构 `colleges`
--
CREATE TABLE IF NOT EXISTS `colleges` (
 `college_id` int(8) NOT NULL auto_increment COMMENT '学院编号自动增加',
 `name` varchar(40) NOT NULL COMMENT '学院名称',
 PRIMARY KEY (`college_id`),
 UNIQUE KEY `name` (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=gb2312 AUTO_INCREMENT=7 ;
--
-- 导出表中的数据 `colleges`
--
INSERT INTO `colleges` (`college_id`, `name`) VALUES
(4, '化学与化工学院'),
(1, '计算机与信息科学学院'),
(6, '美术学院'),
(2, '文学院'),
(5, '音乐学院'),
(3, '政治与公共管理学院');
--
-- 表的结构 `majors`
--
CREATE TABLE IF NOT EXISTS `majors` (
 `major_id` int(8) NOT NULL auto_increment COMMENT '专业号，自动增加',
 `name` varchar(40) NOT NULL COMMENT '专业名',
 `college_id` int(8) default NULL COMMENT '所在学院',
 `counsellor_id` int(10) default NULL COMMENT '辅导员',
 PRIMARY KEY (`major_id`),
 UNIQUE KEY `college_id` (`college_id`,`counsellor_id`)
) ENGINE=InnoDB DEFAULT CHARSET=gb2312 AUTO_INCREMENT=8 ;
--
-- 导出表中的数据 `majors`
--
INSERT INTO `majors` (`major_id`, `name`, `college_id`, `counsellor_id`) VALUES
(1, '电子商务', 1, 1),
(2, '音乐视唱', 5, 1),
(3, '古典文学', 2, 1),
(4, '计算机科学', 1, NULL),
(5, '自动化', 1, NULL),
(6, '现代文学', 2, NULL),
(7, '新闻写作', 2, NULL);

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