assets/Last-Query.html-DpjN7irj.js

import{_ as e,o as t,c as a,e as s}from"./app-Bx8hKGcu.js";const n={},r=s(`<h2 id="最新点查询" tabindex="-1"><a class="header-anchor" href="#最新点查询"><span>最新点查询</span></a></h2><p>最新点查询是时序数据库 Apache IoTDB 中提供的一种特殊查询，最新点查询返回时间序列中时间戳最大的数据点，即一条序列的最新状态。用户可以通过 <code>select last</code> 指定查询最新点。由于最新点数据表征了当前状态，因此在物联网数据分析场景中尤为重要。为了提供毫秒级的返回速度，Apache IoTDB 对最新点查询进行了缓存优化**，**满足用户对设备实时监控的性能需求。</p><p>SQL 语法：</p><div class="language-sql line-numbers-mode" data-ext="sql" data-title="sql"><pre class="language-sql"><code><span class="token keyword">select</span> <span class="token keyword">last</span> <span class="token operator">&lt;</span>Path<span class="token operator">&gt;</span> <span class="token punctuation">[</span>COMMA <span class="token operator">&lt;</span>Path<span class="token operator">&gt;</span><span class="token punctuation">]</span><span class="token operator">*</span> <span class="token keyword">from</span> <span class="token operator">&lt;</span> PrefixPath <span class="token operator">&gt;</span> <span class="token punctuation">[</span>COMMA <span class="token operator">&lt;</span> PrefixPath <span class="token operator">&gt;</span><span class="token punctuation">]</span><span class="token operator">*</span> <span class="token operator">&lt;</span>whereClause<span class="token operator">&gt;</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><p>其含义是：查询时间序列 prefixPath.path 中最近时间戳的数据。</p><p><code>whereClause</code> 中当前只支持含有 <code>&gt;</code> 或 <code>&gt;=</code> 的时间过滤条件，任何其他过滤条件都将会返回异常。</p><p>结果集为四列的结构：</p><div class="language-text line-numbers-mode" data-ext="text" data-title="text"><pre class="language-text"><code>+----+----------+-----+--------+
|Time|timeseries|value|dataType|
+----+----------+-----+--------+
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p><strong>示例 1：</strong> 查询 root.ln.wf01.wt01.status 的最新数据点</p><div class="language-text line-numbers-mode" data-ext="text" data-title="text"><pre class="language-text"><code>IoTDB&gt; select last status from root.ln.wf01.wt01
+-----------------------------+------------------------+-----+--------+
|                         Time|              timeseries|value|dataType|
+-----------------------------+------------------------+-----+--------+
|2017-11-07T23:59:00.000+08:00|root.ln.wf01.wt01.status|false| BOOLEAN|
+-----------------------------+------------------------+-----+--------+
Total line number = 1
It costs 0.000s
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p><strong>示例 2：</strong> 查询 root.ln.wf01.wt01 下 status，temperature 时间戳大于等于 2017-11-07T23:50:00 的最新数据点。</p><div class="language-text line-numbers-mode" data-ext="text" data-title="text"><pre class="language-text"><code>IoTDB&gt; select last status, temperature from root.ln.wf01.wt01 where time &gt;= 2017-11-07T23:50:00
+-----------------------------+-----------------------------+---------+--------+
|                         Time|                   timeseries|    value|dataType|
+-----------------------------+-----------------------------+---------+--------+
|2017-11-07T23:59:00.000+08:00|     root.ln.wf01.wt01.status|    false| BOOLEAN|
|2017-11-07T23:59:00.000+08:00|root.ln.wf01.wt01.temperature|21.067368|  DOUBLE|
+-----------------------------+-----------------------------+---------+--------+
Total line number = 2
It costs 0.002s
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div>`,12),l=[r];function i(o,d){return t(),a("div",null,l)}const p=e(n,[["render",i],["__file","Last-Query.html.vue"]]),u=JSON.parse('{"path":"/zh/UserGuide/V0.13.x/Query-Data/Last-Query.html","title":"","lang":"zh-CN","frontmatter":{"description":"最新点查询 最新点查询是时序数据库 Apache IoTDB 中提供的一种特殊查询，最新点查询返回时间序列中时间戳最大的数据点，即一条序列的最新状态。用户可以通过 select last 指定查询最新点。由于最新点数据表征了当前状态，因此在物联网数据分析场景中尤为重要。为了提供毫秒级的返回速度，Apache IoTDB 对最新点查询进行了缓存优化**，...","head":[["link",{"rel":"alternate","hreflang":"en-us","href":"https://iotdb.apache.org/UserGuide/V0.13.x/Query-Data/Last-Query.html"}],["meta",{"property":"og:url","content":"https://iotdb.apache.org/zh/UserGuide/V0.13.x/Query-Data/Last-Query.html"}],["meta",{"property":"og:site_name","content":"IoTDB Website"}],["meta",{"property":"og:description","content":"最新点查询 最新点查询是时序数据库 Apache IoTDB 中提供的一种特殊查询，最新点查询返回时间序列中时间戳最大的数据点，即一条序列的最新状态。用户可以通过 select last 指定查询最新点。由于最新点数据表征了当前状态，因此在物联网数据分析场景中尤为重要。为了提供毫秒级的返回速度，Apache IoTDB 对最新点查询进行了缓存优化**，..."}],["meta",{"property":"og:type","content":"article"}],["meta",{"property":"og:locale","content":"zh-CN"}],["meta",{"property":"og:locale:alternate","content":"en-US"}],["meta",{"property":"og:updated_time","content":"2023-07-10T03:11:17.000Z"}],["meta",{"property":"article:modified_time","content":"2023-07-10T03:11:17.000Z"}],["script",{"type":"application/ld+json"},"{\\"@context\\":\\"https://schema.org\\",\\"@type\\":\\"Article\\",\\"headline\\":\\"\\",\\"image\\":[\\"\\"],\\"dateModified\\":\\"2023-07-10T03:11:17.000Z\\",\\"author\\":[]}"]]},"headers":[{"level":2,"title":"最新点查询","slug":"最新点查询","link":"#最新点查询","children":[]}],"git":{"createdTime":1688958677000,"updatedTime":1688958677000,"contributors":[{"name":"CritasWang","email":"critas@outlook.com","commits":1}]},"readingTime":{"minutes":1.51,"words":452},"filePathRelative":"zh/UserGuide/V0.13.x/Query-Data/Last-Query.md","localizedDate":"2023年7月10日","autoDesc":true}');export{p as comp,u as data};