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案例涉及到数据库，数据库设计如下：
首先创建一个test数据库，内容如下：
create table if not exists `province` ( `province_id` int(2) not null auto_increment, `province_name` varchar(20) not null, primary key (`province_id`)) engine=innodb default charset=utf8 auto_increment=3 ; insert into `province` (`province_id`, `province_name`) values(1, '安徽'),(2, '浙江'); create table if not exists `city` ( `city_id` int(4) not null auto_increment, `city_name` varchar(20) not null, `province_id` int(4) not null, primary key (`city_id`)) engine=innodb default charset=utf8 auto_increment=5 ; insert into `city` (`city_id`, `city_name`, `province_id`) values(1, '合肥', 1),(2, '安庆', 1),(3, '南京', 2),(4, '徐州', 2); create table if not exists `county` ( `county_id` int(4) not null auto_increment, `county_name` varchar(20) not null, `city_id` int(4) not null, primary key (`county_id`)) engine=innodb default charset=utf8 auto_increment=5 ; insert into `county` (`county_id`, `county_name`, `city_id`) values(1, '怀宁', 2),(2, '望江', 2),(3, '肥东', 1),(4, '肥西', 1);
对数据库说明：我创建了三张表，分别是省（province），市（city）,县（county）,插入了几条测试数据，当然你也可以设计一张表，效率当然没一张表好，所以不建议使用，看你个人习惯。
实现过程并不是很难，思路如下：
1) 初始化所有的省份，这个可以直接从数据库中查询出来省份
2）当用户选择省份的时候触发事件，把当前的省份的id通过ajax发出请求传递到服务端的程序中
3）服务端根据客户端的请求，查询数据库，并按照一定的格式返回给客户端
4）客户端获取服务端的数据，进行必要的处理显示出来
创建select.php (代码简陋，只是实现功能而已，说明原理即可！)
1 <!doctype html public "-//w3c//dtd html 4.01 transitional//en" "http://www.w3.org/tr/html4/loose.dtd">2 <html>3 <head>4 <title>三级联动(作者：mckee - www.phpddt.com)</title>5 <meta http-equiv="content-type"content="text/html; charset=utf-8"/>6 <script>7 function createajax(){8 var xmlhttp = false;9 if (window.xmlhttprequest){10 xmlhttp = new xmlhttprequest();11 }else if(window.activexobject){12 try{13 xmlhttp = new activexobject("msxml2.xmlhttp");14 }catch(e){15 try{16 xmlhttp = new activexobject("microsoft.xmlhttp");17 }catch(e){18 xmlhttp = false;19 }20 }21 }22 return xmlhttp; 23 }24 25 var ajax = null;26 function getcity(province_id){27 ajax = createajax();28 ajax.onreadystatechange=function(){29 if(ajax.readystate == 4){30 if(ajax.status == 200){ 31 var cities = ajax.responsexml.getelementsbytagname("city");32 $('city').length = 0;33 var myoption = document.createelement("option");34 myoption.value = "";35 myoption.innertext = "--请选择城市--";36 $('city').appendchild(myoption);37 for(var i=0;i<cities.length;i++){38 var city_id = cities[i].childnodes[0].childnodes[0].nodevalue;39 var city_name = cities[i].childnodes[1].childnodes[0].nodevalue;40 var myoption = document.createelement("option");41 myoption.value = city_id;42 myoption.innertext = city_name;43 $('city').appendchild(myoption);44 }45 }46 }47 }48 49 ajax.open("post","selectpro.php",true);50 ajax.setrequestheader("content-type", "application/x-www-form-urlencoded");51 ajax.send("province_id="+province_id);52 53 }54 55 function getcounty(city_id){56 ajax = createajax();57 ajax.onreadystatechange=function(){58 if(ajax.readystate == 4){59 if(ajax.status == 200){60 61 var cities = ajax.responsexml.getelementsbytagname("county");62 $('county').length = 0;63 var myoption = document.createelement("option");64 myoption.value = "";65 myoption.innertext = "--请选择县--";66 $('county').appendchild(myoption);67 for(var i=0;i<cities.length;i++){68 var city_id = cities[i].childnodes[0].childnodes[0].nodevalue;69 var city_name = cities[i].childnodes[1].childnodes[0].nodevalue;70 var myoption = document.createelement("option");71 myoption.value = city_id;72 myoption.innertext = city_name;73 $('county').appendchild(myoption);74 }75 }76 }77 }78 ajax.open("post","selectpro.php",true);79 ajax.setrequestheader("content-type", "application/x-www-form-urlencoded");80 ajax.send("city_id="+city_id);81 }82 83 function $(id){84 return document.getelementbyid(id);85 }86 87 </script>88 </head> 89 <body>90 <form name="location">91 <select name="province" onchange="getcity(this.value)">92 <option value="">-- 请选择省份--</option>93 <?php94 $conn = mysql_connect("localhost","root","");95 mysql_select_db("test");96 mysql_query("set names utf8");97 $sql = "select * from province"; 98 $result = mysql_query( $sql ); 99 while($res = mysql_fetch_assoc($result)){ 100 ?> 101 <option value="<?php echo $res['province_id']; ?>"><?php echo $res['province_name']?></option> 102 <?php103 } 104 ?>105 </select>106 107 <select name="city" id="city" onchange="getcounty(this.value)">108 <option value="">--请选择城市--</option>109 </select>110 111 <select name="county" id="county">112 <option value="">--请选择县--</option>113 </select>114 </form>115 </body>116 </html>
创建selectpro.php页面：
117 <?php118 //禁止缓存(www.phpddt.com原创)119 header("content-type:text/xml; charset=utf-8");120 header("cache-control:no-cache");121 //数据库连接122 $conn = mysql_connect("localhost","root","");123 mysql_select_db("test");124 mysql_query("set names utf8");125 126 if(!empty($_post['province_id'])){127 128 $province_id = $_post['province_id'];129 $sql = "select * from city where province_id = {$province_id}";130 $query = mysql_query($sql);131 $info = "<province>";132 while($res = mysql_fetch_assoc($query)){133 $info .= "<city>";134 $info .= "<city_id>".$res['city_id']."</city_id>";135 $info .= "<city_name>".$res['city_name']."</city_name>";136 $info .= "</city>";137 }138 $info .= "</province>";139 echo $info;140 }141 142 if(!empty($_post['city_id'])){143 144 $city_id = $_post['city_id'];145 $sql = "select * from county where city_id = {$city_id}";146 $query = mysql_query($sql);147 $info = "<city>";148 while($res = mysql_fetch_assoc($query)){149 $info .= "<county>";150 $info .= "<county_id>".$res['county_id']."</county_id>";151 $info .= "<county_name>".$res['county_name']."</county_name>";152 $info .= "</county>";153 }154 $info .= "</city>";155 echo $info;156 }157 ?>
界面如下：
以上就是ajax php如何实现三级联动的详细内容。