navicat自动递增的原理

Posted 2023-04-25

为什么我设置递增后删除一个数据再增加一个数据，他的递增字段不是前前一个值+1，而不是被删除数据的递增字段+1

参考技术A 种子在增长以后就表示被用过了，比如说7已经被用过了，但是被你删了，所以在新增时就会从8开始，如果想还是从7开始，就得改种子的值从7开始追问

怎么改呢？

追答

为啥要改？是不是强迫症作祟？如果非要改百度上查查就行了

本回答被提问者采纳 参考技术B 自增字段一旦自增后，就算你删了这个id的数据，自增值还是按照之前的顺序继续自增的，已经使用过的值是不会再次出现的

课程信息管理系统

1.环境的配置：eclipse+tomcat+MySQL+Navicat Premium

2.在Navicat Premium中新建db_book数据库，其中新建tb_books数据表。包含四个字段：id、name、teacher、workplace，将id设为自动递增，否则后面递增会出错。

3.新建web项目，点击两次next，勾选自动生成web.xml。

生成的项目结构如下：

 

 将连接mysql的驱动jar包（mysql-connector-java-8.0.13-bin.jar）复制到WEB-INF下的lib目录下，直接拖拽即可。

4.新建3个包，在每个包下建对应的Java类。

 Book.java

package example.bean.book;  
public class Book { 
 
  private int id; 
  
  private String name; 
  
  private String teacher; 
  
  private String workplace;
public int getId() {
    return id;
}
public void setId(int id) {
    this.id = id;
}
public String getName() {
    return name;
}
public void setName(String name) {
    this.name = name;
}
public String getTeacher() {
    return teacher;
}
public void setTeacher(String teacher) {
    this.teacher = teacher;
}
public String getWorkplace() {
    return workplace;
}
public void setWorkplace(String workplace) {
    this.workplace = workplace;
}

}

 

BookJdbcDao.java

package example.dao.book; 
  
import java.sql.Connection; 
import java.sql.PreparedStatement; 
import java.sql.ResultSet; 
import java.sql.SQLException; 
  
import example.bean.book.Book; 
  
public class BookJdbcDao { 
    
  private PreparedStatement ptmt = null; 
  private ResultSet rs = null; 
  
  public BookJdbcDao() { 
  } 
    
  public void findAll(Connection conn) throws SQLException 
  { 
    //to do    
  } 
    
  public void delete(Connection conn, int id) throws SQLException 
  { 
    String sql = "delete from tb_books where id=?"; 
    try{ 
      ptmt = conn.prepareStatement(sql); 
      // 对SQL语句中的第一个占位符赋值 
      ptmt.setInt(1, id); 
      // 执行更新操作 
      ptmt.executeUpdate(); 
        
    }finally{ 
      if (null!=ptmt) { 
        ptmt.close(); 
      } 
        
      if (null!=conn) { 
        conn.close(); 
      } 
        
    } 
      
  } 
    
  public void update(Connection conn, int id ) throws SQLException 
  { 
    //to do 
      
  } 
}

ConnectionFactory.java

package example.dao.book; 
  
import java.sql.Connection; 
import java.sql.DriverManager; 
import java.sql.SQLException; 
  
public class ConnectionFactory { 
    
  private String driverClassName = "com.mysql.cj.jdbc.Driver"; 
  private String url = "jdbc:mysql://localhost:3306/db_book?&useSSL=false&serverTimezone=UTC"; 
  private String userName = "root"; 
  private String password = "password"; 
    
  private static ConnectionFactory connectionFactory=null; 
  
  private ConnectionFactory() { 
      
    try { 
      Class.forName(driverClassName); 
    } catch (ClassNotFoundException e) { 
      e.printStackTrace(); 
    } 
  } 
    
  public Connection getConnection() throws SQLException 
  { 
    return DriverManager.getConnection(url, userName, password); 
      
  } 
    
  public static ConnectionFactory getInstance() 
  {   
    if (null==connectionFactory) { 
      connectionFactory=new ConnectionFactory(); 
    } 
    return connectionFactory; 
      
  } 
}

DeleteServlet.java

package example.servlet.book; 
import java.io.IOException; 
import java.sql.Connection; 
import javax.servlet.ServletException; 
import javax.servlet.http.HttpServlet; 
import javax.servlet.http.HttpServletRequest; 
import javax.servlet.http.HttpServletResponse; 
import example.dao.book.BookJdbcDao; 
import example.dao.book.ConnectionFactory; 
  
/** 
 * Servlet implementation class DeleteServlet 
 */
public class DeleteServlet extends HttpServlet { 
  private static final long serialVersionUID = 1L; 
  
  /** 
   * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse 
   *   response) 
   */
  protected void doGet(HttpServletRequest request, 
      HttpServletResponse response) throws ServletException, IOException { 
    int id = Integer.valueOf(request.getParameter("id")); 
    try { 
//     
      BookJdbcDao bookDao=new BookJdbcDao(); 
      Connection conn=ConnectionFactory.getInstance().getConnection(); 
      bookDao.delete(conn,id); 
        
    } catch (Exception e) { 
      e.printStackTrace(); 
    } 
    // 重定向到FindServlet 
    response.sendRedirect("FindServlet"); 
  } 
  
  /** 
   * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse 
   *   response) 
   */
  protected void doPost(HttpServletRequest request, 
      HttpServletResponse response) throws ServletException, IOException { 
    doGet(request, response); 
  } 
  
}

FindServlet.java

package example.servlet.book;  
import java.io.IOException; 
import java.sql.Connection; 
import java.sql.DriverManager; 
import java.sql.ResultSet; 
import java.sql.Statement; 
import java.util.ArrayList; 
import java.util.List; 
  
import javax.servlet.ServletException; 
import javax.servlet.http.HttpServlet; 
import javax.servlet.http.HttpServletRequest; 
import javax.servlet.http.HttpServletResponse; 
  
import example.bean.book.Book;//导入包下的Book类
  
/** 
 * Servlet implementation class FindServlet 
 */
public class FindServlet extends HttpServlet { 
  private static final long serialVersionUID = 1L; 
  
  /** 
   * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse 
   *   response) 
   */
  protected void doGet(HttpServletRequest request, 
      HttpServletResponse response) throws ServletException, IOException { 
    try { 
      // 加载数据库驱动，注册到驱动管理器 
      Class.forName("com.mysql.cj.jdbc.Driver"); 
      // 数据库连接字符串 
      String url = "jdbc:mysql://localhost:3306/db_book?&useSSL=false&serverTimezone=UTC"; 
      // 数据库用户名 
      String username = "root"; 
      // 数据库密码 
      String password = "password"; 
      // 创建Connection连接 
      Connection conn = DriverManager.getConnection(url, username, 
          password); 
      // 添加图书信息的SQL语句 
      String sql = "select * from tb_books"; 
      // 获取Statement 
      Statement statement = conn.createStatement(); 
  
      ResultSet resultSet = statement.executeQuery(sql); 
  
      List<Book> list = new ArrayList<Book>(); 
      while (resultSet.next()) { 
  
        Book book = new Book(); 
        book.setId(resultSet.getInt("id")); 
        book.setName(resultSet.getString("name")); 
        book.setTeacher(resultSet.getString("teacher")); 
        book.setWorkplace(resultSet.getString("workplace")); 
        list.add(book); 
      } 
      request.setAttribute("list", list); 
      resultSet.close(); 
      statement.close(); 
      conn.close(); 
  
    } catch (Exception e) { 
      e.printStackTrace(); 
    } 
  
    request.getRequestDispatcher("book_list.jsp") 
        .forward(request, response); 
  
  } 
  
  /** 
   * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse 
   *   response) 
   */
  protected void doPost(HttpServletRequest request, 
      HttpServletResponse response) throws ServletException, IOException { 
    // TODO Auto-generated method stub 
    doGet(request, response); 
  } 
  
}

UpdateServlet.java

package example.servlet.book; 
  
import java.io.IOException; 
import java.sql.Connection; 
import java.sql.DriverManager; 
import java.sql.PreparedStatement; 
  
import javax.servlet.ServletException; 
import javax.servlet.http.HttpServlet; 
import javax.servlet.http.HttpServletRequest; 
import javax.servlet.http.HttpServletResponse; 
  
/** 
 * Servlet implementation class UpdateServlet 
 */
public class UpdateServlet extends HttpServlet { 
  private static final long serialVersionUID = 1L; 
  
  /** 
   * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse 
   *   response) 
   */
  protected void doGet(HttpServletRequest request, 
      HttpServletResponse response) throws ServletException, IOException { 
      request.setCharacterEncoding("UTF-8");
        response.setContentType("text/html;charset=UTF-8");
    int id = Integer.valueOf(request.getParameter("id"));
    String name = request.getParameter("name"); 
    String teacher = request.getParameter("teacher"); 
    String workplace = request.getParameter("workplace"); 
    try { 
      // 加载数据库驱动，注册到驱动管理器 
      Class.forName("com.mysql.cj.jdbc.Driver"); 
      // 数据库连接字符串 
      String url = "jdbc:mysql://localhost:3306/db_book?&useSSL=false&serverTimezone=UTC"; 
      // 数据库用户名 
      String username = "root"; 
      // 数据库密码 
      String password = "password"; 
      // 创建Connection连接 
      Connection conn = DriverManager.getConnection(url, username, 
          password); 
      // 更新SQL语句 
      String sql = "UPDATE tb_books SET name=?,teacher=?,workplace=? WHERE id=?"; 
      // 获取PreparedStatement 
      PreparedStatement ps = conn.prepareStatement(sql); 
      // 对SQL语句中的第一个参数赋值 
      ps.setString(1, name); 
      ps.setString(2, teacher); 
      ps.setString(3, workplace);
      ps.setInt(4, id);
      // 对SQL语句中的第二个参数赋值 
     
      // 执行更新操作 
      ps.executeUpdate(); 
      // 关闭PreparedStatement 
      ps.close(); 
      // 关闭Connection 
      conn.close(); 
    } catch (Exception e) { 
      e.printStackTrace(); 
    } 
    // 重定向到FindServlet 
    response.sendRedirect("FindServlet"); 
  
  } 
  
  /** 
   * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse 
   *   response) 
   */
  protected void doPost(HttpServletRequest request, 
      HttpServletResponse response) throws ServletException, IOException { 
    // TODO Auto-generated method stub 
    doGet(request, response); 
  } 
  
}

5.右击Webcontent新建5个jsp页面。

addbook.jsp

<%@ page language="java" contentType="text/html; charset=utf-8"
  pageEncoding="utf-8"%> 
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 
  
<%@page import="java.sql.Connection"%> 
<%@page import="java.sql.DriverManager"%> 
<%@page import="java.sql.PreparedStatement"%> 
<html> 
<head> 
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 
<title>添加结果</title> 
</head> 
<body> 
  <% 
    request.setCharacterEncoding("utf-8"); 
  %> 
  <jsp:useBean id="book" class="example.bean.book.Book"></jsp:useBean> 
  <jsp:setProperty property="*" name="book" /> 
  <% 
    try { 
      // 加载数据库驱动，注册到驱动管理器 
      Class.forName("com.mysql.cj.jdbc.Driver"); 
      // 数据库连接字符串 
      String url = "jdbc:mysql://localhost:3306/db_book?&useSSL=false&serverTimezone=UTC"; 
      // 数据库用户名 
      String username = "root"; 
      // 数据库密码 
      String password = "password"; 
      // 创建Connection连接 
      Connection conn = DriverManager.getConnection(url, username, 
          password); 
      // 添加图书信息的SQL语句 
      String sql = "insert into tb_books(name,teacher,workplace) values(?,?,?)"; 
      // 获取PreparedStatement 
      PreparedStatement ps = conn.prepareStatement(sql); 
      // 对SQL语句中的第1个参数赋值 
      ps.setString(1, book.getName()); 
      // 对SQL语句中的第2个参数赋值 
      ps.setString(2, book.getTeacher()); 
      ps.setString(3, book.getWorkplace()); 
      // 对SQL语句中的第3个参数赋值 
      // 对SQL语句中的第4个参数赋值 
      // 执行更新操作，返回所影响的行数 
      int row = ps.executeUpdate(); 
      // 判断是否更新成功 
      if (row > 0) { 
        // 更新成输出信息 
        out.print("成功添加了 " + row + "条数据！"); 
      } 
      // 关闭PreparedStatement，释放资源 
      ps.close(); 
      // 关闭Connection，释放资源 
      conn.close(); 
    } catch (Exception e) { 
      out.print("课程信息添加失败！"); 
      e.printStackTrace(); 
    } 
  %> 
  <br> 
  <a href="main.jsp">返回</a> 
</body> 
</html>

book_list.jsp

<!--<%@page import="sun.awt.SunHints.Value"%>-->
<%@ page language="java" contentType="text/html; charset=utf-8"
    pageEncoding="utf-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

<%@page import="java.util.List"%>
<%@page import="example.bean.book.Book"%>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>所有课程信息</title>
<style type="text/css">
td {
    font-size: 12px;
}

h2 {
    margin: 0px
}
</style>
</head>
<body>
    <table align="center" width="450" border="1" height="180"
        bordercolor="white" bgcolor="black" cellpadding="1" cellspacing="1">
        <tr bgcolor="white">
            <td align="center" colspan="7">
                <h2>所有课程信息</h2>
            </td>
        </tr>
        <tr align="center" bgcolor="#e1ffc1">
            <td><b>ID</b></td>
            <td><b>课程名称</b></td>
            <td><b>老师</b></td>
            <td><b>上课地点</b></td>
            <td><b>删除</b></td>
        </tr>
        <%
            // 获取图书信息集合 
            List<Book> list = (List<Book>) request.getAttribute("list");
            // 判断集合是否有效 
            if (list == null || list.size() < 1) {
                out.print("没有数据！");
            } else {
                // 遍历图书集合中的数据 
                for (Book book : list) {
        %>
        <tr align="center" bgcolor="white">
            <td><%=book.getId()%></td>
            <td><%=book.getName()%></td>
            <td><%=book.getTeacher()%></td>
            <td><%=book.getWorkplace()%></td>
            <td><a href="DeleteServlet?id=<%=book.getId()%>">删除</a></td>


        </tr>
        <%
            }
            }
        %>
    </table>
    <h2 align="center">
        <a href="main.jsp">返回主菜单</a>
    </h2>

</body>
</html>

Update.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>课程信息修改</title>
</head>
<script type="text/javascript">
function check(form) {
        with (form) {
            if (name.value == "") { 
                alert("课程名称不能为空"); 
                return false; 
              }   
              if (teacher.value == "") { 
                alert("老师名字不能为空"); 
                return false; 
              } 
              if (workplace.value == "") { 
                alert("上课地点不能为空"); 
                return false; 
              } 
        }
    }
</script>
<body>
<td>
                <form style="align: center; background-color: gray"
                    action="UpdateServlet" method="post" onsubmit=" return check(this);">
                    <input type="text" name="id" size=3> 
                    <input type="text" name="name" size="3">
                    <input type="text" name="teacher" size="3">
                    <input type="text" name="workplace" size="3">
                     <input type="submit" value="修改">
                </form>
            </td>
            <h2 align="center">
        <a href="main.jsp">返回主菜单</a>
    </h2>
</body>
</html>

index.jsp

<%@page import="java.sql.SQLException"%> 
<%@page import="java.sql.DriverManager"%> 
<%@page import="java.sql.Connection"%> 
<%@ page language="java" contentType="text/html; charset=utf-8"
  pageEncoding="utf-8"%> 
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 
<html> 
<head> 
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 
<title>添加课程信息</title> 
  
<script type="text/javascript"> 
  function check(form) { 
    with (form) { 
      if (name.value == "") { 
        alert("课程名称不能为空"); 
        return false; 
      } 
  
      if (teacher.value == "") { 
        alert("老师名字不能为空"); 
        return false; 
      } 
  
      if (workplace.value == "") { 
        alert("上课地点不能为空"); 
        return false; 
      } 
  
    } 
  } 
</script> 
  
</head> 
  
  
<body> 
  
<form action="addbook.jsp" method="post" onsubmit="check(this)"> 
  <table align="center" width="450"> 
    <tr> 
      <td align="center" colspan="2"> 
        <h2>添加课程信息</h2> 
        <hr> 
      </td> 
    </tr> 
  
    <tr> 
      <td align="right">课程名称：</td> 
      <td><input type="text" name="name"></td> 
    </tr> 
  
    <tr> 
      <td align="right">老师：</td> 
      <td><input type="text" name="teacher"></td> 
    </tr> 
 
    <tr> 
      <td align="right">上课地点：</td> 
      <td><input type="text" name="workplace" /></td> 
    </tr> 
    <tr> 
      <td align="center" colspan="2"><input type="submit" value="添　加"> 
      </td> 
    </tr> 
  </table> 
</form> 
  
<h2 align="center">
        <a href="main.jsp">返回主菜单</a>
    </h2>
  
</body> 
</html>

main.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>课程信息管理系统</title>
</head>
<body>
<center><h1>主菜单</h1><center>
<table><center>
<td><A href="index.jsp"><font size=2>课程信息录入</font></A></td>
<td><A href="Update.jsp"><font size=2>课程信息修改</font></A></td>
<h2 align="center"> 
  <a href="FindServlet">查询课程信息</a> 
</h2> 
<h2 align="center"> 
  <a href="FindServlet">删除课程信息</a> 
</h2> 
</table></center>
</body>
</html>

接下来配置web.xml

<?xml version="1.0" encoding="UTF-8"?> 
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" id="WebApp_ID" version="2.5"> 
 <display-name>JdbcConnection</display-name> 
 <welcome-file-list> 
  <welcome-file>index.html</welcome-file> 
  <welcome-file>index.htm</welcome-file> 
  <welcome-file>index.jsp</welcome-file> 
  <welcome-file>default.html</welcome-file> 
  <welcome-file>default.htm</welcome-file> 
  <welcome-file>default.jsp</welcome-file> 
 </welcome-file-list> 
 <servlet> 
  <description></description> 
  <display-name>FindServlet</display-name> 
  <servlet-name>FindServlet</servlet-name> 
  <servlet-class>example.servlet.book.FindServlet</servlet-class> 
 </servlet> 
 <servlet-mapping> 
  <servlet-name>FindServlet</servlet-name> 
  <url-pattern>/FindServlet</url-pattern> 
 </servlet-mapping> 
 <servlet> 
  <description></description> 
  <display-name>UpdateServlet</display-name> 
  <servlet-name>UpdateServlet</servlet-name> 
  <servlet-class>example.servlet.book.UpdateServlet</servlet-class> 
 </servlet> 
 <servlet-mapping> 
  <servlet-name>UpdateServlet</servlet-name> 
  <url-pattern>/UpdateServlet</url-pattern> 
 </servlet-mapping> 
 <servlet> 
  <description></description> 
  <display-name>DeleteServlet</display-name> 
  <servlet-name>DeleteServlet</servlet-name> 
  <servlet-class>example.servlet.book.DeleteServlet</servlet-class> 
 </servlet> 
 <servlet-mapping> 
  <servlet-name>DeleteServlet</servlet-name> 
  <url-pattern>/DeleteServlet</url-pattern> 
 </servlet-mapping> 
</web-app>

代码复制完成之后，需要将连接数据库的用户名密码修改为连接自己的数据库的用户名和密码。