| import{_ as e,o as n,c as s,a,f as t}from"./app-saYZgPSe.js";const i={},r=t(`<h2 id="最新点查询" tabindex="-1"><a class="header-anchor" href="#最新点查询" aria-hidden="true">#</a> 最新点查询</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"><pre class="language-sql"><code><span class="token keyword">select</span> <span class="token keyword">last</span> <span class="token operator"><</span>Path<span class="token operator">></span> <span class="token punctuation">[</span>COMMA <span class="token operator"><</span>Path<span class="token operator">></span><span class="token punctuation">]</span><span class="token operator">*</span> <span class="token keyword">from</span> <span class="token operator"><</span> PrefixPath <span class="token operator">></span> <span class="token punctuation">[</span>COMMA <span class="token operator"><</span> PrefixPath <span class="token operator">></span><span class="token punctuation">]</span><span class="token operator">*</span> <span class="token operator"><</span>whereClause<span class="token operator">></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>></code> 或 <code>>=</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"><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"><pre class="language-text"><code>IoTDB> 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"><pre class="language-text"><code>IoTDB> select last status, temperature from root.ln.wf01.wt01 where time >= 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);function o(l,d){return n(),s("div",null,[a(` |
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| regarding copyright ownership. The ASF licenses this file |
| to you under the Apache License, Version 2.0 (the |
| "License"); you may not use this file except in compliance |
| with the License. You may obtain a copy of the License at |
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| software distributed under the License is distributed on an |
| "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY |
| KIND, either express or implied. See the License for the |
| specific language governing permissions and limitations |
| under the License. |
| |
| `),r])}const p=e(i,[["render",o],["__file","Last-Query.html.vue"]]);export{p as default}; |
