49 lines
1.3 KiB
Markdown
49 lines
1.3 KiB
Markdown
---
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{
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"title": "NDV",
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"language": "zh-CN"
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}
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---
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<!--
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Licensed to the Apache Software Foundation (ASF) under one
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or more contributor license agreements. See the NOTICE file
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distributed with this work for additional information
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regarding copyright ownership. The ASF licenses this file
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to you under the Apache License, Version 2.0 (the
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"License"); you may not use this file except in compliance
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with the License. You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing,
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software distributed under the License is distributed on an
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"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
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KIND, either express or implied. See the License for the
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specific language governing permissions and limitations
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under the License.
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-->
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# NDV
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## description
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### Syntax
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`NDV(expr)`
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返回类似于 COUNT(DISTINCT col) 结果的近似值聚合函数。
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它比 COUNT 和 DISTINCT 组合的速度更快，并使用固定大小的内存，因此对于高基数的列可以使用更少的内存。
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## example
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```
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MySQL > select ndv(query_id) from log_statis group by datetime;
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+-----------------+
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| ndv(`query_id`) |
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+-----------------+
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| 17721 |
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+-----------------+
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```
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##keyword
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NDV
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