<form method="post" action="crud.php">
    <table>
        <tr>
<!--            <td>-->
<!--                学号<input type="text" name="id">-->
<!--            </td>-->

            <td>
                &nbsp;&nbsp;姓名<input type="text" name="name">
            </td>

            <td>
                &nbsp;&nbsp;性别
                <select name="sex">
　　　　　　　　　　 　<option value="">请选择</option>
                    <option value="1">男</option>
                    <option value="0">女</option>
                </select>
            </td>

            <td>
                &nbsp;&nbsp;出生年月<input type="text" name="birth_start">——<input type="text" name="birth_end">
            </td>

            <td>
                &nbsp;&nbsp;班级<input type="text" name="class">
            </td>

            <td>
                <input type="submit" value="查一查">
            </td>
        </tr>
    </table>
</form>

if (!empty($_POST)){
//    $id = $_POST['id'];
    $name = $_POST['name'];
    $sex = $_POST['sex'];
    $birth_start= $_POST['birth_start'];
    $birth_end = $_POST['birth_end'];
    $class = $_POST['class'];

//1
//    if (!empty($name)){
//        $sql .= "and name like '%{$name}%'";
//    }

//2
//    $sql .= !empty($id) ? "and Sno like '%{$id}%'" : "";

    $sql .= !empty($name)? "and Sname like '%{$name}%' " : "";
    $sql .= ($sex!="")? "and Ssex = '{$sex}' " : "";
    if (!empty($birth_start)&&!empty($birth_end)){
        $sql .= "and Sbirthday >'{$birth_start}' and Sbirthday <'{$birth_end}' ";
    }
    $sql .= !empty($class)? "and Class like '%{class}%' " : "";

}
$data = $db->query($sql)->fetch_all();

?>