后端找数据库

一、连接数据库

二、查找  resultSet只使用于查找

 ResultSet resultSet = statement.executeQuery(sql);

//执行sql的对象调用方法执行sql语句 executeQuery查找 返回查询的结果

三、更新 / 删除 / 插入操作（executeUpdate）

1.删除

• 执行修改类 SQL（update/delete/insert 语句），返回受影响的行数（如 delete 语句返回删除了多少条记录）。

  String sql = "delete from student where id = 4";
  int num = statement.executeUpdate(sql); // num 为 1（删除 1 条记录）

2.删除记录

四、修改

五、总体代码

package db.java.copy;public class test {public static void main(String[] args) {}//前段找后端 后端找数据库//查找  resultSet只使用于查找public static String search(String sql) {//加载驱动 trytry {//sql注入Class.forName("com.mysql.jdbc.Driver");//加载 MySQL JDBC 驱动程序；固定写法String url = "jdbc:mysql://localhost:3306/javaweb";//定义数据库url javaweb是表的数据库名字，写法固定String user = "root";//账号密码String password = "root";//驱动管理类调方法进行连接 得到连接对象Connection connection = (Connection) DriverManager.getConnection(url, user, password);//连接对象调用方法 创建执行sql的对象Statement statement = (Statement) connection.createStatement();//执行sql的对象调用方法执行sql语句 executeQuery查找 返回查询的结果ResultSet resultSet = statement.executeQuery(sql);//处理结果String res = "[";while (resultSet.next()) {//从表格中提取数据，存储到相应的变量中int id = resultSet.getInt("id");String name = resultSet.getString("name");String sex = resultSet.getString("sex");int age = resultSet.getInt("age");int classid = resultSet.getInt("classid");System.out.println(id + " " + name + " " + sex + " " + age + " " + classid);res += "{\"id\":\"" + id + "\",\"name\":\"" + name + "\",\"sex\":\"" + sex + "\",\"age\":\"" + age + "\",\"classid\":\"" + classid + "\"},";}//打印出提取的数据，储存到res中res = res.substring(0, res.length() - 1);res += "]";//完成JSON数组创建,[ ]//关闭资源if (resultSet != null) {resultSet.close();}if (statement != null) {statement.close();}if (connection != null) {connection.close();}return res;} catch (Exception e) {// TODO Auto-generated catch blocke.printStackTrace();return "";}}// 删除public static int delete(String sql) {try {Class.forName("com.mysql.jdbc.Driver");String url = "jdbc:mysql://localhost:3306/javaweb";String user = "root";String password = "root";//使用DriverManager获取与数据库的连接对象。Connection connection = (Connection) DriverManager.getConnection(url, user, password);//通过connection对象创建一个 Statement对象，用于执行 SQL语句。Statement statement = (Statement) connection.createStatement();//通过  statement  对象执行传入的 SQL 更新语句，并返回受影响的行数。int num = statement.executeUpdate(sql);if (statement != null) {statement.close();}if (connection != null) {connection.close();}return num;} catch (Exception e) {e.printStackTrace();return 0;}}//修改public static int update(String sql) {try {Class.forName("com.mysql.jdbc.Driver");String url = "jdbc:mysql://localhost:3306/javaweb";String user = "root";String password = "root";Connection connection = (Connection) DriverManager.getConnection(url, user, password);Statement statement = (Statement) connection.createStatement();int num = statement.executeUpdate(sql);if (statement != null) {statement.close();}if (connection != null) {connection.close();}return num;} catch (Exception e) {e.printStackTrace();return 0;}}public static void main(String[] args) {/*String sql = "select * from class";  //查询 class 表的所有记录 String[] columns = {"id","name"};//调用MysqlUtil.getJsonBySql(sql, columns)方法,  * 将查询结果转换为 JSON格式，并将结果存储在res变量中。String res = MysqlUtil.getJsonBySql(sql, columns);System.out.println(res);*/    String sql = "delete from student where id = 4";//从student表中，删除 id = 4 的记录int num = MysqlUtil.del(sql);//调用 MysqlUtil工具类的del方法，执行上述删除 SQL。System.out.println(num);//直接调方法删除就可以}}}