php如何实现按条件查询

<?php //单查询
require "DBDA.class.php";//调取封装类
$db=new DBDA;
$Sname="";//创建变量，为了后面可以让Sname在表单中显示
$sql="select * from t_student";

if(!empty($_POST["Sname"]))//确定是否存在数据
{
 $Sname=$_POST["Sname"];
 $sql="select * from t_student where Sname like '%{$Sname}%' ";//模糊查询
}
?>

<form action="chaxun.php" method="post"><!--因为查询数据在当前页面，所以提交到当前页面-->
<div>姓名：<input type="text" name="Sname" value="<?php echo $Sname ?>" />
<input type="submit" value="查询" /></div>
</form><br />

<table width="100% " border="1" cellpadding="0" cellspacing="0">
 <tr>
 <td>姓名</td>
 <td>性别</td>
 <td>班级</td>
 </tr>

<?php
$arr=$db->query($sql);
foreach($arr as $v)
{
 $str = str_replace($Sname,"<span style='color:red'>{$Sname}</span>",$v[1]);//用span标签使关键字变为红色，
 echo "<tr>
 <td>{$str}</td>
 <td>{$v[2]}</td>
 <td>{$v[4]}</td>
 </tr>"; 
}
?>

<table width="100%" border="1" cellpadding="0" cellspacing="0">
<tr>
 <td>代号</td>
 <td>名称</td>
 <td>系列</td>
 <td>上市时间</td>
 <td>价格</td>
</tr>
</table>

<?php
require "DBDA.class.php";
$db=new DBDA();//1.如果没有提交数据，显示所有//2.如果有提交数据，根据关键字查询显示$name="";
$tj1=" 1=1 "; //第一个条件，对应名称，要用空格隔开$tj2=" 1=1 ";//第二个条件，对应系列，要用空格隔开if(!empty($_POST["name"]))
{
 $name = $_POST["name"];
 $tj1 = " name like '%{$name}%' ";
}if(!empty($_POST["brand"]))
{
 $brand = $_POST["brand"];
 $tj2 = " brand = '{$brand}' ";
}//总条件$tj=" {$tj1}and{$tj2} ";
$sql="select * from car where".$tj;?>

<?php
$arr = $db->query($sql);foreach($arr as $v)
{
 $str = str_replace($name,"<span style='color:red'>{$name}</span>",$v[1]);
 echo "<tr>
 <td>{$v[0]}</td>
 <td>{$str}</td>
 <td>{$v[2]}</td>
 <td>{$v[3]}</td>
 <td>{$v[7]}</td>
</tr>";}?>