使用多线程在50w数据表alarm_point中提取id(String)，dev_name(String)两个字段，然后根据id将dev_name的数据更新至字段alm_class_desc(String)中，数据库为clickhouse，输出sql语句，使用java完成

您可以参考以下Java代码实现：

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class UpdateAlarmClassDesc {

    private static final String URL = "jdbc:clickhouse://localhost:8123/database";
    private static final String USERNAME = "username";
    private static final String PASSWORD = "password";
    private static final String SELECT_SQL = "SELECT id, dev_name FROM alarm_point";
    private static final String UPDATE_SQL = "UPDATE alarm_point SET alm_class_desc = ? WHERE id = ?";

    public static void main(String[] args) {
        ExecutorService executorService = Executors.newFixedThreadPool(10);
        try (Connection conn = DriverManager.getConnection(URL, USERNAME, PASSWORD);
             PreparedStatement selectStmt = conn.prepareStatement(SELECT_SQL);
             ResultSet rs = selectStmt.executeQuery()) {
            while (rs.next()) {
                String id = rs.getString("id");
                String devName = rs.getString("dev_name");
                executorService.submit(() -> updateAlarmClassDesc(conn, id, devName));
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            executorService.shutdown();
        }
    }

    private static void updateAlarmClassDesc(Connection conn, String id, String devName) {
        try (PreparedStatement updateStmt = conn.prepareStatement(UPDATE_SQL)) {
            updateStmt.setString(1, devName);
            updateStmt.setString(2, id);
            updateStmt.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

这段代码使用Java的多线程技术，从clickhouse数据库的`alarm_point`表中提取`id`和`dev_name`字段，并使用`id`更新`alm_class_desc`字段。可以根据需要修改线程池的大小和数据库连接信息。