php+mysql中的一些语句

mysql建立一个表格：
create table hyhtbsc(
id int(4) primary key auto_increment,
username varchar(20),
password varchar(20)
);
查询数据库内容：
<?php
$conn = mysql_connect("localhost","root","pass");
mysql_select_db("mydb",$conn);
$sql = "select * from mytable";
$result = mysql_query($sql,$conn);
while($row = mysql_fetch_array($result)){
echo "<td>".$row['myfield']."</td>";
}
mysql_free_result($result);
mysql_close($conn);
?>
添加数据库内容：
<?php
$id=trim($_POST['id']);
$conn = mysql_connect("localhost","root","pass");
mysql_select_db("mydb", $conn);
if($id != ''){
$sql="INSERT INTO ggs_chapter (id) VALUES ('".$book_id."')";
mysql_query($sql,$conn);

echo '添加成功！';
}
?>
<form name="add_book" action="" method="post">
<table>
<tr><td>
<select name="id">
<?php
$sql="select * from table";
$result=mysql_query($sql,$conn);
while($row = mysql_fetch_array($result)){
echo "<option value=\"".$row['id']."\">".$row['name']."</option>";
}
mysql_free_result($result);
mysql_close($conn);
?>