解决PreparedStatement中动态SQL运算符的MySQL语法错误

String sql = "SELECT * FROM total WHERE totals ? ?;";
PreparedStatement stmt = con.prepareStatement(sql);
stmt.setString(1,signString); // 尝试绑定运算符
stmt.setString(2,amount);    // 绑定值

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

public class DynamicSqlOperatorExample {

    public static void main(String[] args) {
        // 假设type和amount是从其他地方获取的动态值
        int type = 1; // 1: greater, 2: lesser, 3: equal
        String amount = "100";

        Connection con = null;
        PreparedStatement stmt = null;
        ResultSet rs = null;

        try {
            Class.forName("com.mysql.cj.jdbc.Driver"); // 注意：新版本MySQL驱动类名为com.mysql.cj.jdbc.Driver
            con = DriverManager.getConnection("jdbc:mysql://localhost:3306/jsupermarket", "root", "");

            String signString = "";
            if (type == 1) { // greater
                signString = ">=";
                System.out.print("1 selected \n");
            } else if (type == 2) { // lesser
                signString = "<=";
                System.out.print("2 selected \n");
            } else if (type == 3) { // equal
                signString = "=";
                System.out.print("3 selected \n");
            }

            // 核心修改：将运算符直接拼接进SQL字符串，而值仍然使用占位符
            // 注意：SQL语句末尾不应包含分号
            String sql = "SELECT * FROM total WHERE totals " + signString + " ?";

            stmt = con.prepareStatement(sql);
            stmt.setString(1, amount); // 只有值才通过占位符绑定

            rs = stmt.executeQuery();

            // 处理结果集
            while (rs.next()) {
                // 示例：打印total列的值
                System.out.println("Found total: " + rs.getString("totals"));
            }

        } catch (ClassNotFoundException e) {
            System.err.println("JDBC Driver not found: " + e.getMessage());
        } catch (SQLException e) {
            System.err.println("SQL Error: " + e.getMessage());
            e.printStackTrace();
        } finally {
            // 关闭资源
            try {
                if (rs != null) rs.close();
                if (stmt != null) stmt.close();
                if (con != null) con.close();
            } catch (SQLException e) {
                System.err.println("Error closing resources: " + e.getMessage());
            }
        }
    }
}