java通过id查询数据库的基本信息 jdbc根据id查询信息

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数据小探 2024-02-20 11:33:33

文章标签 java通过id查询数据库的基本信息 sql servlet 数据库 System 文章分类 Java 后端开发

查询&结果集处理

按照下标取值

package com.qf.jdbc;

import java.sql.*;

public class Demo1_select {

    public static void main(String[] args) throws Exception {
        Class.forName("com.mysql.jdbc.Driver");
        Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/java2212?useSSL=false","root","123456");
        Statement s = conn.createStatement( );

        // 执行查询使用executeQuery
        String sql = "select sal,hiredate,empno id,ename name from emp";

        // 查询返回的是一种虚拟表,用集合装数据.即ResultSet
        ResultSet rs = s.executeQuery(sql);
        while(rs.next()){
            /**
             * 取值有两种方式:
             *    getXxx(),根据数据类型再按以下方式取值
             *   1 按照列下标,从1开始,与虚拟表列顺序一致
             *   2 按照列名,是虚拟表的名字.即如果有别名就是别名取值
             */
            // 列下标取值
            int empno = rs.getInt(1);
            String ename = rs.getString(2);
            double sal = rs.getDouble(3);
            Date date = rs.getDate(4);
            System.out.println(empno+"-"+ename+"-"+sal+"-"+date);
        }

        // 结果集关流
        rs.close();
        s.close();
        conn.close();
    }

}

按照列名取值（重点）

package com.qf.jdbc;

import java.sql.*;

public class Demo1_select {

    public static void main(String[] args) throws Exception {
        Class.forName("com.mysql.jdbc.Driver");
        Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/java2212?useSSL=false","root","123456");
        Statement s = conn.createStatement( );

        // 执行查询使用executeQuery
        String sql = "select sal,hiredate,empno id,ename name from emp";

        // 查询返回的是一种虚拟表,用集合装数据.即ResultSet
        ResultSet rs = s.executeQuery(sql);
        while(rs.next()){
            /**
             * 取值有两种方式:
             *    getXxx(),根据数据类型再按以下方式取值
             *   1 按照列下标,从1开始,与虚拟表列顺序一致
             *   2 按照列名,是虚拟表的名字.即如果有别名就是别名取值
             */
            // 列名取值
            int empno = rs.getInt("id");
            String ename = rs.getString("name");
            double sal = rs.getDouble("sal");
            Date hiredate = rs.getDate("hiredate");

            System.out.println(empno+"-"+ename+"-"+sal+"-"+hiredate);

        }

        // 结果集关流
        rs.close();
        s.close();
        conn.close();
    }
}

我们建议以后就使用列名取值!

登录案例

准备数据表

CREATE TABLE `user` (
  `uid` int(11) NOT NULL AUTO_INCREMENT,
  `username` varchar(255) DEFAULT NULL,
  `password` varchar(255) DEFAULT NULL,
  `createTime` date DEFAULT NULL,
  PRIMARY KEY (`uid`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;
 
package com.qf.jdbc;

import java.sql.*;
import java.util.Scanner;


public class Demo2_Login {

    public static void main(String[] args) throws Exception {
        Scanner scanner = new Scanner(System.in);
        System.out.println("==== 欢迎登录天地银行 ====" );
        System.out.println("请输入用户名: " );
        String username = scanner.next( );

        System.out.println("请输入密码: " );
        String password = scanner.next( );

        System.out.print("正在登录" );
        for (int i = 1; i < 4; i++) {
            Thread.sleep(1000);
            System.out.print(". ");
        }
        System.out.println( );

        Class.forName("com.mysql.jdbc.Driver");
        Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/java2212?useSSL=false","root","123456");
        Statement s = conn.createStatement( );

        // 登录
        String sql = "select * from user where username = '"+username+"' and password = '"+password+"'";

        ResultSet rs = s.executeQuery(sql);

        if(rs.next()) {
            int uid = rs.getInt("uid");
            String uname = rs.getString("username");
            String pwd = rs.getString("password");
            Date createTime = rs.getDate("createTime");

            System.out.println("登录成功!欢迎"+uname+",您的注册时间是:"+createTime );

        } else {
            System.out.println("用户名或密码错误!" );
        }

        rs.close();
        s.close();
        conn.close();
        scanner.close();
    }
}

预处理语句（重点）

将之前的处理语句变为预处理语句parameterStatement

package com.qf.jdbc;

import java.sql.*;
import java.util.Scanner;
public class Demo3_LoginPlus {

    public static void main(String[] args) throws Exception {
        Scanner scanner = new Scanner(System.in);
        System.out.println("==== 欢迎登录天地银行 ====" );
        System.out.println("请输入用户名: " );
        String username = scanner.nextLine( );

        System.out.println("请输入密码: " );
        String password = scanner.nextLine( );

        System.out.print("正在登录" );
        for (int i = 1; i < 4; i++) {
            Thread.sleep(1000);
            System.out.print(". ");
        }
        System.out.println( );

        Class.forName("com.mysql.jdbc.Driver");
        Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/java2212?useSSL=false","root","123456");
        // Statement是处理语句,它有可能出现SQL注入
        // Statement s = conn.createStatement( );

        // 登录
        String sql = "select * from user where username = ? and password = ?";
        /**
         * SQL字符串的模板是: 将拼接处改成占位符?
         * 预处理语句,会将SQL字符串中的模板处理替换
         * 使用ps对象,给?处赋值
         */
        PreparedStatement ps = conn.prepareStatement(sql);
        // parameterIndex,即?号的位置
        ps.setString(1,username);
        ps.setString(2,password);

        ResultSet rs = ps.executeQuery();

        if(rs.next()) {
            int uid = rs.getInt("uid");
            String uname = rs.getString("username");
            String pwd = rs.getString("password");
            Date createTime = rs.getDate("createTime");

            System.out.println("登录成功!欢迎"+uname+",您的注册时间是:"+createTime );

        } else {
            System.out.println("用户名或密码错误!" );
        }

        rs.close();
        ps.close();
        conn.close();
        scanner.close();
    }
}

使用预处理语句，完成注册案例

package com.qf.jdbc;

import java.sql.*;
import java.util.Scanner;
public class Demo4_regist {

    public static void main(String[] args) throws Exception {
        Scanner scanner = new Scanner(System.in);
        System.out.println("==== 欢迎登录天地银行[注册系统] ====" );
        System.out.println("请输入用户名: " );
        String username = scanner.nextLine( );

        System.out.println("请输入密码: " );
        String password = scanner.nextLine( );

        System.out.print("正在注册..." );


        Class.forName("com.mysql.jdbc.Driver");
        Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/java2212?useSSL=false","root","123456");

        // 登录
        String sql = "insert into user (username,password,createTime) values (?,?,?)";
        /**
         * SQL字符串的模板是: 将拼接处改成占位符?
         * 预处理语句,会将SQL字符串中的模板处理替换
         * 使用ps对象,给?处赋值
         */
        PreparedStatement ps = conn.prepareStatement(sql);
        // parameterIndex,即?号的位置
        ps.setString(1,username);
        ps.setString(2,password);
        /**
         * java.util.Date提供了设置年与日的构造方法
         * 但是年需要减去 1900
         * 月份是从0-11
         */
        ps.setDate(3,new Date(new java.util.Date(90,0,1).getTime()));

        int i = ps.executeUpdate( );
        if (i > 0) {
            System.out.println("注册成功");
        }
        
        ps.close();
        conn.close();
        scanner.close();
    }
}

ORM（重点，理解）

Object Relation Mapping 对象关系映射

1 要根据表设计类

package com.qf.model;

import java.util.Date;
public class Emp {

    private int empno;
    private String ename;
    private String job;
    private int mgr;
    private Date hiredate;
    private double sal;
    private double comm;
    private int deptno;

    @Override
    public String toString() {
        return "Emp{" +
                "empno=" + empno +
                ", ename='" + ename + '\'' +
                ", job='" + job + '\'' +
                ", mgr=" + mgr +
                ", hiredate=" + hiredate +
                ", sal=" + sal +
                ", comm=" + comm +
                ", deptno=" + deptno +
                '}';
    }

    public Emp(int empno, String ename, String job, int mgr, Date hiredate, double sal, double comm, int deptno) {
        this.empno = empno;
        this.ename = ename;
        this.job = job;
        this.mgr = mgr;
        this.hiredate = hiredate;
        this.sal = sal;
        this.comm = comm;
        this.deptno = deptno;
    }

    public Emp() {
    }

    public int getEmpno() {
        return empno;
    }

    public void setEmpno(int empno) {
        this.empno = empno;
    }

    public String getEname() {
        return ename;
    }

    public void setEname(String ename) {
        this.ename = ename;
    }

    public String getJob() {
        return job;
    }

    public void setJob(String job) {
        this.job = job;
    }

    public int getMgr() {
        return mgr;
    }

    public void setMgr(int mgr) {
        this.mgr = mgr;
    }

    public Date getHiredate() {
        return hiredate;
    }

    public void setHiredate(Date hiredate) {
        this.hiredate = hiredate;
    }

    public double getSal() {
        return sal;
    }

    public void setSal(double sal) {
        this.sal = sal;
    }

    public double getComm() {
        return comm;
    }

    public void setComm(double comm) {
        this.comm = comm;
    }

    public int getDeptno() {
        return deptno;
    }

    public void setDeptno(int deptno) {
        this.deptno = deptno;
    }
}

2 写查询需求

package com.qf.jdbc;

import com.qf.model.Emp;

import java.sql.*;
import java.util.ArrayList;

public class Demo5_select_orm {

    public static void main(String[] args) throws Exception {
        Class.forName("com.mysql.jdbc.Driver");
        Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/java2212?useSSL=false","root","123456");

        String sql = "select * from emp";
        PreparedStatement ps = conn.prepareStatement(sql);

        ResultSet rs = ps.executeQuery();

        // 创建集合,存储所有对象
        ArrayList<Emp> emps = new ArrayList<>( );

        while(rs.next()){
            int empno = rs.getInt("empno");
            String ename = rs.getString("ename");
            double sal = rs.getDouble("sal");
            int mgr = rs.getInt("mgr");
            double comm = rs.getDouble("comm");
            int deptno = rs.getInt("deptno");
            Date hiredate = rs.getDate("hiredate");
            String job = rs.getString("job");
            // 封装对象
            Emp emp = new Emp(empno,ename,job,mgr,hiredate,sal,comm,deptno);
            // 添加集合
            emps.add(emp);
        }
        
        for (Emp emp : emps) {
            System.out.println(emp );
        }

        // 结果集关流
        rs.close();
        ps.close();
        conn.close();
    }
}

DBUtil（能用）

package com.qf.util;

import java.sql.*;

public class DBUtil {

    private static final String URL = "jdbc:mysql://localhost:3306/java2212?useSSL=false";
    private static final String USERNAME = "root";
    private static final String PASSWORD = "123456";


    static {
        try {
            Class.forName("com.mysql.jdbc.Driver");
        } catch (ClassNotFoundException e) {
            e.printStackTrace( );
        }
    }

    public static Connection getConnection() {
        Connection conn = null;
        try {
            conn = DriverManager.getConnection(URL, USERNAME, PASSWORD);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return conn;
    }

    public static void closeAll(Statement s,Connection conn){
        try {
            s.close();
            conn.close();
        } catch (Exception e) {
            e.printStackTrace( );
        }
    }

    public static void closeAll( ResultSet rs, Statement s,Connection conn){
        try {
            rs.close();
            s.close();
            conn.close();
        } catch (Exception e) {
            e.printStackTrace( );
        }
    }
}

使用DBUtil

package com.qf.jdbc;

import com.qf.model.Emp;
import com.qf.util.DBUtil;

import java.sql.*;
import java.util.ArrayList;

public class Demo6_select_orm_dbutil {

    public static void main(String[] args) throws Exception {

        // 获得连接对象
        Connection conn = DBUtil.getConnection( );
        String sql = "select * from emp";
        PreparedStatement ps = conn.prepareStatement(sql);
       //执行sql操作
        ResultSet rs = ps.executeQuery();

        // 创建集合,存储所有对象
        ArrayList<Emp> emps = new ArrayList<>( );

        while(rs.next()){
            int empno = rs.getInt("empno");
            String ename = rs.getString("ename");
            double sal = rs.getDouble("sal");
            int mgr = rs.getInt("mgr");
            double comm = rs.getDouble("comm");
            int deptno = rs.getInt("deptno");
            Date hiredate = rs.getDate("hiredate");
            String job = rs.getString("job");
            //创建emp 对象
            Emp emp = new Emp(empno,ename,job,mgr,hiredate,sal,comm,deptno);
            //把获得的数据存到集合中
            emps.add(emp);
        }
        //遍历集合
        for (Emp emp : emps) {
            System.out.println(emp );
        }

        // 结果集关流
        DBUtil.closeAll(rs,ps,conn);
    }
}

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