查询数据

查询数据

主要查询功能

TDengine 采用 SQL 作为查询语言。应用程序可以通过 REST API 或连接器发送 SQL 语句，用户还可以通过 TDengine 命令行工具 taos 手动执行 SQL 即席查询（Ad-Hoc Query）。TDengine 支持如下查询功能：

单列、多列数据查询
标签和数值的多种过滤条件：>, <, =, <>, like 等
聚合结果的分组（Group by）、排序（Order by）、约束输出（Limit/Offset）
时间窗口（Interval）、会话窗口（Session）和状态窗口（State_window）等窗口切分聚合查询
数值列及聚合结果的四则运算
时间戳对齐的连接查询（Join Query: 隐式连接）操作
多种聚合/计算函数: count, max, min, avg, sum, twa, stddev, leastsquares, top, bottom, first, last, percentile, apercentile, last_row, spread, diff 等

例如：在命令行工具 taos 中，从表 d1001 中查询出 voltage > 215 的记录，按时间降序排列，仅仅输出 2 条。

taos> select * from d1001 where voltage > 215 order by ts desc limit 2;
           ts            |       current        |   voltage   |        phase         |
======================================================================================
 2018-10-03 14:38:16.800 |             12.30000 |         221 |              0.31000 |
 2018-10-03 14:38:15.000 |             12.60000 |         218 |              0.33000 |
Query OK, 2 row(s) in set (0.001100s)

为满足物联网场景的需求，TDengine 支持几个特殊的函数，比如 twa(时间加权平均)，spread (最大值与最小值的差)，last_row(最后一条记录)等，更多与物联网场景相关的函数将添加进来。

具体的查询语法请看 TDengine SQL 的数据查询章节。

多表聚合查询

物联网场景中，往往同一个类型的数据采集点有多个。TDengine 采用超级表(STable)的概念来描述某一个类型的数据采集点，一张普通的表来描述一个具体的数据采集点。同时 TDengine 使用标签来描述数据采集点的静态属性，一个具体的数据采集点有具体的标签值。通过指定标签的过滤条件，TDengine 提供了一高效的方法将超级表(某一类型的数据采集点)所属的子表进行聚合查询。对普通表的聚合函数以及绝大部分操作都适用于超级表，语法完全一样。

示例一

在 TDengine CLI，查找加利福尼亚州所有智能电表采集的电压平均值，并按照 location 分组。

taos> SELECT AVG(voltage), location FROM meters GROUP BY location;
       avg(voltage)        |                             location                             |
===============================================================================================
             219.200000000 | California.SanFrancisco                                          |
             221.666666667 | California.LosAngeles                                            |
Query OK, 2 rows in database (0.005995s)

示例二

在 TDengine CLI, 查找 groupId 为 2 的所有智能电表的记录条数，电流的最大值。

taos> SELECT count(*), max(current) FROM meters where groupId = 2;
     cunt(*)  |    max(current)  |
==================================
            5 |             13.4 |
Query OK, 1 row(s) in set (0.002136s)

在 TDengine SQL 的数据查询一章，查询类操作都会注明是否支持超级表。

降采样查询、插值

物联网场景里，经常需要通过降采样（down sampling）将采集的数据按时间段进行聚合。TDengine 提供了一个简便的关键词 interval 让按照时间窗口的查询操作变得极为简单。比如，将智能电表 d1001 采集的电流值每 10 秒钟求和

taos> SELECT _wstart, sum(current) FROM d1001 INTERVAL(10s);
         _wstart         |       sum(current)        |
======================================================
 2018-10-03 14:38:00.000 |              10.300000191 |
 2018-10-03 14:38:10.000 |              24.900000572 |
Query OK, 2 rows in database (0.003139s)

降采样操作也适用于超级表，比如：将加利福尼亚州所有智能电表采集的电流值每秒钟求和

taos> SELECT _wstart, SUM(current) FROM meters where location like "California%" INTERVAL(1s);
         _wstart         |       sum(current)        |
======================================================
 2018-10-03 14:38:04.000 |              10.199999809 |
 2018-10-03 14:38:05.000 |              23.699999809 |
 2018-10-03 14:38:06.000 |              11.500000000 |
 2018-10-03 14:38:15.000 |              12.600000381 |
 2018-10-03 14:38:16.000 |              34.400000572 |
Query OK, 5 rows in database (0.007413s)

降采样操作也支持时间偏移，比如：将所有智能电表采集的电流值每秒钟求和，但要求每个时间窗口从 500 毫秒开始

taos> SELECT _wstart, SUM(current) FROM meters INTERVAL(1s, 500a);
         _wstart         |       sum(current)        |
======================================================
 2018-10-03 14:38:03.500 |              10.199999809 |
 2018-10-03 14:38:04.500 |              10.300000191 |
 2018-10-03 14:38:05.500 |              13.399999619 |
 2018-10-03 14:38:06.500 |              11.500000000 |
 2018-10-03 14:38:14.500 |              12.600000381 |
 2018-10-03 14:38:16.500 |              34.400000572 |
Query OK, 6 rows in database (0.005515s)

物联网场景里，每个数据采集点采集数据的时间是难同步的，但很多分析算法(比如 FFT)需要把采集的数据严格按照时间等间隔的对齐，在很多系统里，需要应用自己写程序来处理，但使用 TDengine 的降采样操作就轻松解决。

如果一个时间间隔里，没有采集的数据，TDengine 还提供插值计算的功能。

语法规则细节请见 TDengine SQL 的按时间窗口切分聚合章节。

示例代码

查询数据

在 SQL 写入一章，我们创建了 power 数据库，并向 meters 表写入了一些数据，以下示例代码展示如何查询这个表的数据。

Java
Python
Go
Rust
Node.js
C#
C
PHP

package com.taos.example;
import java.sql.*;
public class RestQueryExample {
    private static Connection getConnection() throws SQLException {
        String jdbcUrl = "jdbc:TAOS-RS://localhost:6041/power?user=root&password=taosdata";
        return DriverManager.getConnection(jdbcUrl);
    }
    private static void printRow(ResultSet rs) throws SQLException {
        ResultSetMetaData meta = rs.getMetaData();
        for (int i = 1; i <= meta.getColumnCount(); i++) {
            String value = rs.getString(i);
            System.out.print(value);
            System.out.print("\t");
        }
        System.out.println();
    }
    private static void printColName(ResultSet rs) throws SQLException {
        ResultSetMetaData meta = rs.getMetaData();
        for (int i = 1; i <= meta.getColumnCount(); i++) {
            String colLabel = meta.getColumnLabel(i);
            System.out.print(colLabel);
            System.out.print("\t");
        }
        System.out.println();
    }
    private static void processResult(ResultSet rs) throws SQLException {
        printColName(rs);
        while (rs.next()) {
            printRow(rs);
        }
    }
    private static void queryData() throws SQLException {
        try (Connection conn = getConnection()) {
            try (Statement stmt = conn.createStatement()) {
                ResultSet rs = stmt.executeQuery("SELECT AVG(voltage) FROM meters GROUP BY location");
                processResult(rs);
            }
        }
    }
    public static void main(String[] args) throws SQLException {
        queryData();
    }
}
// possible output:
// avg(voltage) location
// 222.0    California.LosAngeles
// 219.0    California.SanFrancisco

查看源码

通过迭代逐行获取查询结果。

def query_api_demo(conn: taos.TaosConnection):
    result: taos.TaosResult = conn.query("SELECT tbname, * FROM meters LIMIT 2")
    print("field count:", result.field_count)
    print("meta of fields[1]:", result.fields[1])
    print("======================Iterate on result=========================")
    for row in result:
        print(row)
# field count: 7
# meta of fields[1]: {name: ts, type: 9, bytes: 8}
# ======================Iterate on result=========================
# ('d1003', datetime.datetime(2018, 10, 3, 14, 38, 5, 500000), 11.800000190734863, 221, 0.2800000011920929, 'california.losangeles', 2)
# ('d1003', datetime.datetime(2018, 10, 3, 14, 38, 16, 600000), 13.399999618530273, 223, 0.28999999165534973, 'california.losangeles', 2)

查看源码

一次获取所有查询结果，并把每一行转化为一个字典返回。

def fetch_all_demo(conn: taos.TaosConnection):
    result: taos.TaosResult = conn.query("SELECT ts, current FROM meters LIMIT 2")
    rows = result.fetch_all_into_dict()
    print("row count:", result.row_count)
    print("===============all data===================")
    print(rows)
# row count: 2
# ===============all data===================
# [{'ts': datetime.datetime(2018, 10, 3, 14, 38, 5, 500000), 'current': 11.800000190734863},
# {'ts': datetime.datetime(2018, 10, 3, 14, 38, 16, 600000), 'current': 13.399999618530273}]

查看源码

package main
import (
    "database/sql"
    "log"
    "time"
    _ "github.com/taosdata/driver-go/v3/taosRestful"
)
func main() {
    var taosDSN = "root:taosdata@http(localhost:6041)/power"
    taos, err := sql.Open("taosRestful", taosDSN)
    if err != nil {
        log.Fatalln("failed to connect TDengine, err:", err)
    }
    defer taos.Close()
    rows, err := taos.Query("SELECT ts, current FROM meters LIMIT 2")
    if err != nil {
        log.Fatalln("failed to select from table, err:", err)
    }
    defer rows.Close()
    for rows.Next() {
        var r struct {
            ts      time.Time
            current float32
        }
        err := rows.Scan(&r.ts, &r.current)
        if err != nil {
            log.Fatalln("scan error:\n", err)
            return
        }
        log.Println(r.ts, r.current)
    }
}

查看源码

use taos::sync::*;
fn main() -> anyhow::Result<()> {
    let taos = TaosBuilder::from_dsn("ws:///power")?.build()?;
    let mut result = taos.query("SELECT ts, current FROM meters LIMIT 2")?;
    // print column names
    let meta = result.fields();
    println!("{}", meta.iter().map(|field| field.name()).join("\t"));
    // print rows
    let rows = result.rows();
    for row in rows {
        let row = row?;
        for (_name, value) in row {
            print!("{}\t", value);
        }
        println!();
    }
    Ok(())
}
// output(suppose you are in +8 timezone):
// ts      current
// 2018-10-03T14:38:05+08:00       10.3
// 2018-10-03T14:38:15+08:00       12.6

查看源码

const taos = require("@tdengine/client");
const conn = taos.connect({ host: "localhost", database: "power" });
const cursor = conn.cursor();
const query = cursor.query("SELECT ts, current FROM meters LIMIT 2");
query.execute().then(function (result) {
  result.pretty();
});
// output:
// Successfully connected to TDengine
//            ts             |         current          |
// =======================================================
// 2018-10-03 14:38:05.000   | 10.3                     |
// 2018-10-03 14:38:15.000   | 12.6                     |

查看源码

using TDengineDriver;
using TDengineDriver.Impl;
using System.Runtime.InteropServices;
namespace TDengineExample
{
    internal class QueryExample
    {
        static void Main()
        {
            IntPtr conn = GetConnection();
            try
            {
                // run query
                IntPtr res = TDengine.Query(conn, "SELECT * FROM meters LIMIT 2");
                if (TDengine.ErrorNo(res) != 0)
                {
                    throw new Exception("Failed to query since: " + TDengine.Error(res));
                }
                // get filed count
                int fieldCount = TDengine.FieldCount(res);
                Console.WriteLine("fieldCount=" + fieldCount);
                // print column names
                List<TDengineMeta> metas = LibTaos.GetMeta(res);
                for (int i = 0; i < metas.Count; i++)
                {
                    Console.Write(metas[i].name + "\t");
                }
                Console.WriteLine();
                // print values
                List<Object> resData = LibTaos.GetData(res);
                for (int i = 0; i < resData.Count; i++)
                {
                    Console.Write($"|{resData[i].ToString()} \t");
                    if (((i + 1) % metas.Count == 0))
                    {
                        Console.WriteLine("");
                    }
                }
                Console.WriteLine();
                // Free result after use
                TDengine.FreeResult(res);
            }
            finally
            {
                TDengine.Close(conn);
            }
        }
        static IntPtr GetConnection()
        {
            string host = "localhost";
            short port = 6030;
            string username = "root";
            string password = "taosdata";
            string dbname = "power";
            var conn = TDengine.Connect(host, username, password, dbname, port);
            if (conn == IntPtr.Zero)
            {
                throw new Exception("Connect to TDengine failed");
            }
            else
            {
                Console.WriteLine("Connect to TDengine success");
            }
            return conn;
        }
    }
}
// output:
// Connect to TDengine success
// fieldCount=6
// ts      current voltage phase   location        groupid
// 1648432611249   10.3    219     0.31    California.SanFrancisco        2
// 1648432611749   12.6    218     0.33    California.SanFrancisco        2

查看源码

// compile with:
// gcc -o query_example query_example.c -ltaos
#include <inttypes.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <taos.h>
typedef int16_t VarDataLenT;
#define TSDB_NCHAR_SIZE sizeof(int32_t)
#define VARSTR_HEADER_SIZE sizeof(VarDataLenT)
#define GET_FLOAT_VAL(x) (*(float *)(x))
#define GET_DOUBLE_VAL(x) (*(double *)(x))
#define varDataLen(v) ((VarDataLenT *)(v))[0]
int printRow(char *str, TAOS_ROW row, TAOS_FIELD *fields, int numFields) {
  int  len = 0;
  char split = ' ';
  for (int i = 0; i < numFields; ++i) {
    if (i > 0) {
      str[len++] = split;
    }
    if (row[i] == NULL) {
      len += sprintf(str + len, "%s", "NULL");
      continue;
    }
    switch (fields[i].type) {
      case TSDB_DATA_TYPE_TINYINT:
        len += sprintf(str + len, "%d", *((int8_t *)row[i]));
        break;
      case TSDB_DATA_TYPE_UTINYINT:
        len += sprintf(str + len, "%u", *((uint8_t *)row[i]));
        break;
      case TSDB_DATA_TYPE_SMALLINT:
        len += sprintf(str + len, "%d", *((int16_t *)row[i]));
        break;
      case TSDB_DATA_TYPE_USMALLINT:
        len += sprintf(str + len, "%u", *((uint16_t *)row[i]));
        break;
      case TSDB_DATA_TYPE_INT:
        len += sprintf(str + len, "%d", *((int32_t *)row[i]));
        break;
      case TSDB_DATA_TYPE_UINT:
        len += sprintf(str + len, "%u", *((uint32_t *)row[i]));
        break;
      case TSDB_DATA_TYPE_BIGINT:
        len += sprintf(str + len, "%" PRId64, *((int64_t *)row[i]));
        break;
      case TSDB_DATA_TYPE_UBIGINT:
        len += sprintf(str + len, "%" PRIu64, *((uint64_t *)row[i]));
        break;
      case TSDB_DATA_TYPE_FLOAT: {
        float fv = 0;
        fv = GET_FLOAT_VAL(row[i]);
        len += sprintf(str + len, "%f", fv);
      } break;
      case TSDB_DATA_TYPE_DOUBLE: {
        double dv = 0;
        dv = GET_DOUBLE_VAL(row[i]);
        len += sprintf(str + len, "%lf", dv);
      } break;
      case TSDB_DATA_TYPE_BINARY:
      case TSDB_DATA_TYPE_NCHAR: {
        int32_t charLen = varDataLen((char *)row[i] - VARSTR_HEADER_SIZE);
        memcpy(str + len, row[i], charLen);
        len += charLen;
      } break;
      case TSDB_DATA_TYPE_TIMESTAMP:
        len += sprintf(str + len, "%" PRId64, *((int64_t *)row[i]));
        break;
      case TSDB_DATA_TYPE_BOOL:
        len += sprintf(str + len, "%d", *((int8_t *)row[i]));
      default:
        break;
    }
  }
  return len;
}
/**
 * @brief print column name and values of each row
 *
 * @param res
 * @return int
 */
static int printResult(TAOS_RES *res) {
  int         numFields = taos_num_fields(res);
  TAOS_FIELD *fields = taos_fetch_fields(res);
  char        header[256] = {0};
  int len = 0;
  for (int i = 0; i < numFields; ++i) {
    len += sprintf(header + len, "%s ", fields[i].name);
  }
  puts(header);
  TAOS_ROW row = NULL;
  while ((row = taos_fetch_row(res))) {
    char temp[256] = {0};
    printRow(temp, row, fields, numFields);
    puts(temp);
  }
}
int main() {
  TAOS *taos = taos_connect("localhost", "root", "taosdata", "power", 6030);
  if (taos == NULL) {
    puts("failed to connect to server");
    exit(EXIT_FAILURE);
  }
  TAOS_RES *res = taos_query(taos, "SELECT * FROM meters LIMIT 2");
  if (taos_errno(res) != 0) {
    printf("failed to execute taos_query. error: %s\n", taos_errstr(res));
    exit(EXIT_FAILURE);
  }
  printResult(res);
  taos_free_result(res);
  taos_close(taos);
  taos_cleanup();
}
// output:
// ts current voltage phase location groupid 
// 1648432611249 10.300000 219 0.310000 California.SanFrancisco 2
// 1648432611749 12.600000 218 0.330000 California.SanFrancisco 2

查看源码

<?php
use TDengine\Connection;
use TDengine\Exception\TDengineException;
try {
    // instantiate
    $host = 'localhost';
    $port = 6030;
    $username = 'root';
    $password = 'taosdata';
    $dbname = 'power';
    $connection = new Connection($host, $port, $username, $password, $dbname);
    // connect
    $connection->connect();
    $resource = $connection->query('SELECT ts, current FROM meters LIMIT 2');
    var_dump($resource->fetch());
} catch (TDengineException $e) {
    // throw exception
    throw $e;
}

查看源码

note

无论是使用 REST 连接还是原生连接的连接器，以上示例代码都能正常工作。
唯一需要注意的是：由于 REST 接口无状态，不能使用 use db 语句来切换数据库。

异步查询

除同步查询 API 之外，TDengine 还提供性能更高的异步调用 API 处理数据插入、查询操作。在软硬件环境相同的情况下，异步 API 处理数据插入的速度比同步 API 快 2-4 倍。异步 API 采用非阻塞式的调用方式，在系统真正完成某个具体数据库操作前，立即返回。调用的线程可以去处理其他工作，从而可以提升整个应用的性能。异步 API 在网络延迟严重的情况下，优点尤为突出。

需要注意的是，只有使用原生连接的连接器，才能使用异步查询功能。

Python
C#
C

import time
from ctypes import *
from taos import *
def fetch_callback(p_param, p_result, num_of_rows):
    print("fetched ", num_of_rows, "rows")
    p = cast(p_param, POINTER(Counter))
    result = TaosResult(p_result)
    if num_of_rows == 0:
        print("fetching completed")
        p.contents.done = True
        result.close()
        return
    if num_of_rows < 0:
        p.contents.done = True
        result.check_error(num_of_rows)
        result.close()
        return None
    for row in result.rows_iter(num_of_rows):
        print(row)
    p.contents.count += result.row_count
    result.fetch_rows_a(fetch_callback, p_param)
def query_callback(p_param, p_result, code):
    if p_result is None:
        return
    result = TaosResult(p_result)
    if code == 0:
        result.fetch_rows_a(fetch_callback, p_param)
    result.check_error(code)
class Counter(Structure):
    _fields_ = [("count", c_int), ("done", c_bool)]
    def __str__(self):
        return "{ count: %d, done: %s }" % (self.count, self.done)
def test_query(conn):
    counter = Counter(count=0)
    conn.query_a("select ts, current, voltage from power.meters", query_callback, byref(counter))
    while not counter.done:
        print(counter)
        time.sleep(1)
    print(counter)
    conn.close()
if __name__ == "__main__":
    test_query(connect())
# possible output:
# { count: 0, done: False }
# fetched  8 rows
# 1538548685000 10.300000 219
# 1538548695000 12.600000 218
# 1538548696800 12.300000 221
# 1538548696650 10.300000 218
# 1538548685500 11.800000 221
# 1538548696600 13.400000 223
# 1538548685500 10.800000 223
# 1538548686500 11.500000 221
# fetched  0 rows
# fetching completed
# { count: 8, done: True }

查看源码

note

这个示例程序，目前在 Windows 系统上还无法运行

using System;
using System.Collections.Generic;
using TDengineDriver;
using TDengineDriver.Impl;
using System.Runtime.InteropServices;
namespace TDengineExample
{
    public class AsyncQueryExample
    {
        static void Main()
        {
            IntPtr conn = GetConnection();
            try
            {
                QueryAsyncCallback queryAsyncCallback = new QueryAsyncCallback(QueryCallback);
                TDengine.QueryAsync(conn, "select * from meters", queryAsyncCallback, IntPtr.Zero);
                Thread.Sleep(2000);
            }
            finally
            {
                TDengine.Close(conn);
            }
        }
        static void QueryCallback(IntPtr param, IntPtr taosRes, int code)
        {
            if (code == 0 && taosRes != IntPtr.Zero)
            {
                FetchRawBlockAsyncCallback fetchRowAsyncCallback = new FetchRawBlockAsyncCallback(FetchRawBlockCallback);
                TDengine.FetchRawBlockAsync(taosRes, fetchRowAsyncCallback, param);
            }
            else
            {
                throw new Exception($"async query data failed,code:{code},reason:{TDengine.Error(taosRes)}");
            }
        }
        // Iteratively call this interface until "numOfRows" is no greater than 0.
        static void FetchRawBlockCallback(IntPtr param, IntPtr taosRes, int numOfRows)
        {
            if (numOfRows > 0)
            {
                Console.WriteLine($"{numOfRows} rows async retrieved");
                IntPtr pdata = TDengine.GetRawBlock(taosRes);
                List<TDengineMeta> metaList = TDengine.FetchFields(taosRes);
                List<object> dataList = LibTaos.ReadRawBlock(pdata, metaList, numOfRows);
                for (int i = 0; i < dataList.Count; i++)
                {
                    if (i != 0 && (i + 1) % metaList.Count == 0)
                    {
                        Console.WriteLine("{0}\t|", dataList[i]);
                    }
                    else
                    {
                        Console.Write("{0}\t|", dataList[i]);
                    }
                }
                Console.WriteLine("");
                TDengine.FetchRawBlockAsync(taosRes, FetchRawBlockCallback, param);
            }
            else
            {
                if (numOfRows == 0)
                {
                    Console.WriteLine("async retrieve complete.");
                }
                else
                {
                    throw new Exception($"FetchRawBlockCallback callback error, error code {numOfRows}");
                }
                TDengine.FreeResult(taosRes);
            }
        }
        static IntPtr GetConnection()
        {
            string host = "localhost";
            short port = 6030;
            string username = "root";
            string password = "taosdata";
            string dbname = "power";
            var conn = TDengine.Connect(host, username, password, dbname, port);
            if (conn == IntPtr.Zero)
            {
                throw new Exception("Connect to TDengine failed");
            }
            else
            {
                Console.WriteLine("Connect to TDengine success");
            }
            return conn;
        }
    }
}
// //output:
// // Connect to TDengine success
// // 8 rows async retrieved
// // 1538548685500   |       11.8    |       221     |       0.28    |       california.losangeles   |       2       |
// // 1538548696600   |       13.4    |       223     |       0.29    |       california.losangeles   |       2       |
// // 1538548685000   |       10.8    |       223     |       0.29    |       california.losangeles   |       3       |
// // 1538548686500   |       11.5    |       221     |       0.35    |       california.losangeles   |       3       |
// // 1538548685000   |       10.3    |       219     |       0.31    |       california.sanfrancisco         |       2       |
// // 1538548695000   |       12.6    |       218     |       0.33    |       california.sanfrancisco         |       2       |
// // 1538548696800   |       12.3    |       221     |       0.31    |       california.sanfrancisco         |       2       |
// // 1538548696650   |       10.3    |       218     |       0.25    |       california.sanfrancisco         |       3       |
// // async retrieve complete.

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/**
 * @brief call back function of taos_fetch_row_a
 *
 * @param param : the third parameter you passed to taos_fetch_row_a
 * @param res : pointer of TAOS_RES
 * @param numOfRow : number of rows fetched in this batch. will be 0 if there is no more data.
 * @return void*
 */
void *fetch_row_callback(void *param, TAOS_RES *res, int numOfRow) {
  printf("numOfRow = %d \n", numOfRow);
  int         numFields = taos_num_fields(res);
  TAOS_FIELD *fields = taos_fetch_fields(res);
  TAOS       *_taos = (TAOS *)param;
  if (numOfRow > 0) {
    for (int i = 0; i < numOfRow; ++i) {
      TAOS_ROW row = taos_fetch_row(res);
      char     temp[256] = {0};
      printRow(temp, row, fields, numFields);
      puts(temp);
    }
    taos_fetch_rows_a(res, fetch_row_callback, _taos);
  } else {
    printf("no more data, close the connection.\n");
    taos_free_result(res);
    taos_close(_taos);
    taos_cleanup();
  }
}
/**
 * @brief callback function of taos_query_a
 *
 * @param param: the fourth parameter you passed to taos_query_a
 * @param res : the result set
 * @param code : status code
 * @return void*
 */
void *select_callback(void *param, TAOS_RES *res, int code) {
  printf("query callback ...\n");
  TAOS *_taos = (TAOS *)param;
  if (code == 0 && res) {
    printHeader(res);
    taos_fetch_rows_a(res, fetch_row_callback, _taos);
  } else {
    printf("failed to execute taos_query. error: %s\n", taos_errstr(res));
    taos_free_result(res);
    taos_close(_taos);
    taos_cleanup();
    exit(EXIT_FAILURE);
  }
}
int main() {
  TAOS *taos = taos_connect("localhost", "root", "taosdata", "power", 6030);
  if (taos == NULL) {
    puts("failed to connect to server");
    exit(EXIT_FAILURE);
  }
  // param one is the connection returned by taos_connect.
  // param two is the SQL to execute.
  // param three is the callback function.
  // param four can be any pointer. It will be passed to your callback function as the first parameter. we use taos
  // here, because we want to close it after getting data.
  taos_query_a(taos, "SELECT * FROM meters", select_callback, taos);
  sleep(1);
}
// output:
// query callback ...
// ts current voltage phase location groupid
// numOfRow = 8
// 1538548685500 11.800000 221 0.280000 california.losangeles 2
// 1538548696600 13.400000 223 0.290000 california.losangeles 2
// 1538548685000 10.800000 223 0.290000 california.losangeles 3
// 1538548686500 11.500000 221 0.350000 california.losangeles 3
// 1538548685000 10.300000 219 0.310000 california.sanfrancisco 2
// 1538548695000 12.600000 218 0.330000 california.sanfrancisco 2
// 1538548696800 12.300000 221 0.310000 california.sanfrancisco 2
// 1538548696650 10.300000 218 0.250000 california.sanfrancisco 3
// numOfRow = 0
// no more data, close the connection.

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