update doc: add jdbc example for data subscribe
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@ -295,6 +295,142 @@ $ taos
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这时，因为电流超过了10A，您应该可以看到示例程序将它输出到了屏幕上。
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您可以继续插入一些数据观察示例程序的输出。
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### jdbc使用数据订阅功能
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（1）使用订阅功能前的数据准备
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```shell
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# 创建power库
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taos> create database power;
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# 切换库
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taos> use power;
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# 创建超级表
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taos> create table meters(ts timestamp, current float, voltage int, phase int) tags(location binary(64), groupI
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d int);
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# 创建表
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taos> create table d1001 using meters tags ("Beijing.Chaoyang",2);
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taos> create table d1002 using meters tags ("Beijing.Haidian",2);
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# 插入测试数据
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taos> insert into d1001 values("2020-08-15 12:00:00.000", 12, 220, 1),("2020-08-15 12:10:00.000", 12.3, 220, 2),("2020-08-15 12:20:00.000", 12.2, 220, 1);
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taos> insert into d1002 values("2020-08-15 12:00:00.000", 9.9, 220, 1),("2020-08-15 12:10:00.000", 10.3, 220, 1),("2020-08-15 12:20:00.000", 11.2, 220, 1);
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# 从超级表meters查询current大于10的数据
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taos> select * from meters where current > 10;
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ts | current | voltage | phase| location | groupid |
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===========================================================================================================
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2020-08-15 12:10:00.000 | 10.30000 | 220 | 1 | Beijing.Haidian | 2 |
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2020-08-15 12:20:00.000 | 11.20000 | 220 | 1 | Beijing.Haidian | 2 |
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2020-08-15 12:00:00.000 | 12.00000 | 220 | 1 | Beijing.Chaoyang | 2 |
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2020-08-15 12:10:00.000 | 12.30000 | 220 | 2 | Beijing.Chaoyang | 2 |
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2020-08-15 12:20:00.000 | 12.20000 | 220 | 1 | Beijing.Chaoyang | 2 |
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Query OK, 5 row(s) in set (0.004896s)
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```
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（2）使用jdbc提供的订阅功能
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```java
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public class SubscribeDemo {
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private static final String topic = "topic_meter_current_bg_10";
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private static final String sql = "select * from meters where current > 10";
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public static void main(String[] args) {
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Connection connection = null;
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Statement statement = null;
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TSDBSubscribe subscribe = null;
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long subscribeId = 0;
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try {
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// 加载驱动
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Class.forName("com.taosdata.jdbc.TSDBDriver");
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// 获取Connectin
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Properties properties = new Properties();
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properties.setProperty(TSDBDriver.PROPERTY_KEY_CHARSET, "UTF-8");
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properties.setProperty(TSDBDriver.PROPERTY_KEY_TIME_ZONE, "UTC-8");
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String jdbcUrl = "jdbc:TAOS://127.0.0.1:6030/power?user=root&password=taosdata";
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connection = DriverManager.getConnection(jdbcUrl, properties);
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System.out.println("create the connection");
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// 创建Subscribe
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subscribe = ((TSDBConnection) connection).createSubscribe();
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// subscribe订阅topic，topic为主题名称，sql为查询语句，restart代表是否每次订阅接受历史数据
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subscribeId = subscribe.subscribe(topic, sql, true);
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System.out.println("create a subscribe topic: " + topic + "@[" + subscribeId + "]");
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int count = 0;
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while (true) {
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// 消费数据
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TSDBResultSet resultSet = subscribe.consume(subscribeId);
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// 打印结果集
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if (resultSet != null) {
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ResultSetMetaData metaData = resultSet.getMetaData();
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while (resultSet.next()) {
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int columnCount = metaData.getColumnCount();
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for (int i = 1; i <= columnCount; i++) {
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System.out.print(metaData.getColumnLabel(i) + " : " + resultSet.getString(i) + "\t");
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}
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System.out.println("\n====================");
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count++;
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}
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}
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if (count > 10)
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break;
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TimeUnit.SECONDS.sleep(1);
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}
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} catch (Exception e) {
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e.printStackTrace();
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} finally {
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try {
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if (null != subscribe && subscribeId != 0) {
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subscribe.unsubscribe(subscribeId, true);
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System.out.println("unsubscribe the top@[" + subscribeId + "]");
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}
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if (statement != null) {
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statement.close();
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System.out.println("close the statement.");
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}
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if (connection != null) {
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connection.close();
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System.out.println("close the connection.");
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}
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} catch (SQLException throwables) {
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throwables.printStackTrace();
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}
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}
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}
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}
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```
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（3）订阅功能演示
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运行demo，首先，subscribe会将满足情况的历史数据消费
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```shell
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# java -jar subscribe.jar
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ts : 1597464000000 current : 12.0 voltage : 220 phase : 1 location : Beijing.Chaoyang groupid : 2
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====================
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ts : 1597464600000 current : 12.3 voltage : 220 phase : 2 location : Beijing.Chaoyang groupid : 2
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====================
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ts : 1597465200000 current : 12.2 voltage : 220 phase : 1 location : Beijing.Chaoyang groupid : 2
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====================
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ts : 1597464600000 current : 10.3 voltage : 220 phase : 1 location : Beijing.Haidian groupid : 2
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====================
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ts : 1597465200000 current : 11.2 voltage : 220 phase : 1 location : Beijing.Haidian groupid : 2
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====================
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```
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接着，使用taos客户端向表中新增数据
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```shell
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# taos
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taos> use power；
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taos> insert into d1001 values("2020-08-15 12:40:00.000", 12.4, 220, 1);
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```
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查看数据消费情况
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```shell
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ts : 1597466400000 current : 12.4 voltage : 220 phase : 1 location : Beijing.Chaoyang groupid : 2
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====================
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```
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## 缓存(Cache)
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