JSP借书系统借完书后用什么方法让剩余图书数量减掉借阅的数量

import java.sql.Connection;
import java.sql.Statement;
import java.text.SimpleDateFormat;
import java.util.Date;
import com.core.DBUtile.DBConnection;

public class jieshuadd {

private String username ;
private String bookid ;
private String deadline ;
private String purpose;
private String borrowno;
private String surplus;


public String DBsql(){
    Date now=new Date();     
    SimpleDateFormat dateFormat=new SimpleDateFormat("yyyy-MM-dd");  
    String dt = dateFormat.format( now ); 

    Connection conn = DBConnection.getOrcalConnection();
    String sql = "INSERT INTO br_info(username,bookno,borrowdate,borrowno,purpose,deadline,reback,backno)"
         + "VALUES('" + username + "','" + bookid + "','" + dt + "'," + borrowno + ",'" + purpose 
         + "','" + deadline + "','n',0)" ;

    //String sql1="SELECT surplus FORM bookinfo";
    //int num==Integer.parseInt(sql1);




String dql2 = "fail";
System.out.println(sql);

try{

    Statement sm = conn.createStatement();
    int rs = sm.executeUpdate(sql);
    if(rs >= 1){
        dql2 = "success";

    }

    sm.close();
    conn.close();






}catch (Exception e) {

}
return dql2;

}

public String getUsername() {
    return username;
}



public void setUsername(String username) {
    this.username = username;
}



public String getBookid() {
    return bookid;
}



public void setBookid(String bookid) {
    this.bookid = bookid;
}



public String getDeadline() {
    return deadline;
}



public void setDeadline(String deadline) {
    this.deadline = deadline;
}



public String getPurpose() {
    return purpose;
}



public void setPurpose(String purpose) {
    this.purpose = purpose;
}



public String getBorrowno() {
    return borrowno;
}



public void setBorrowno(String borrowno) {
    this.borrowno = borrowno;
}



public String getSurplus() {
    return surplus;
}



public void setSurplus(String surplus) {
    this.surplus = surplus;
}

}

http://zhidao.baidu.com/link?url=m2hT2MQW8SQ8gg2my62R1rOoM2DqrUYabHG00rZaHre8eilobsCvRpgWO3YmLfCxwnNjf6tphuvvL7EUxiHuT_
http://blog.csdn.net/yafzhp/article/details/6623065

给数据库加触发器，当有人借一本书后。将当前总书数-1保存到一个记录结果中。

最简单就是给 借书表的添加操作 添加一个触发器。。
当然也可以在JSp 这边处理·