0f0ccdadb1
### What changes were proposed in this pull request? A few improvements to the sql ref SELECT doc: 1. correct the syntax of SELECT query 2. correct the default of null sort order 3. correct the GROUP BY syntax 4. several minor fixes ### Why are the changes needed? refine document ### Does this PR introduce any user-facing change? N/A ### How was this patch tested? N/A Closes #27866 from cloud-fan/doc. Authored-by: Wenchen Fan <wenchen@databricks.com> Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
100 lines
3.5 KiB
Markdown
100 lines
3.5 KiB
Markdown
---
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layout: global
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title: CLUSTER BY Clause
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displayTitle: CLUSTER BY Clause
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license: |
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Licensed to the Apache Software Foundation (ASF) under one or more
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contributor license agreements. See the NOTICE file distributed with
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this work for additional information regarding copyright ownership.
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The ASF licenses this file to You under the Apache License, Version 2.0
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(the "License"); you may not use this file except in compliance with
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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, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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---
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The <code>CLUSTER BY</code> clause is used to first repartition the data based
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on the input expressions and then sort the data within each partition. This is
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semantically equivalent to performing a
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[DISTRIBUTE BY](sql-ref-syntax-qry-select-distribute-by.html) followed by a
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[SORT BY](sql-ref-syntax-qry-select-sortby.html). This clause only ensures that the
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resultant rows are sorted within each partition and does not guarantee a total order of output.
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### Syntax
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{% highlight sql %}
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CLUSTER BY { expression [ , ... ] }
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{% endhighlight %}
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### Parameters
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<dl>
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<dt><code><em>expression</em></code></dt>
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<dd>
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Specifies combination of one or more values, operators and SQL functions that results in a value.
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</dd>
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</dl>
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### Examples
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{% highlight sql %}
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CREATE TABLE person (name STRING, age INT);
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INSERT INTO person VALUES
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('Zen Hui', 25),
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('Anil B', 18),
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('Shone S', 16),
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('Mike A', 25),
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('John A', 18),
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('Jack N', 16);
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-- Reduce the number of shuffle partitions to 2 to illustrate the behavior of `CLUSTER BY`.
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-- It's easier to see the clustering and sorting behavior with less number of partitions.
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SET spark.sql.shuffle.partitions = 2;
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-- Select the rows with no ordering. Please note that without any sort directive, the results
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-- of the query is not deterministic. It's included here to show the difference in behavior
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-- of a query when `CLUSTER BY` is not used vs when it's used. The query below produces rows
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-- where age column is not sorted.
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SELECT age, name FROM person;
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+---+-------+
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|age|name |
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+---+-------+
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|16 |Shone S|
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|25 |Zen Hui|
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|16 |Jack N |
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|25 |Mike A |
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|18 |John A |
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|18 |Anil B |
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+---+-------+
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-- Produces rows clustered by age. Persons with same age are clustered together.
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-- In the query below, persons with age 18 and 25 are in first partition and the
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-- persons with age 16 are in the second partition. The rows are sorted based
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-- on age within each partition.
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SELECT age, name FROM person CLUSTER BY age;
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+---+-------+
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|age|name |
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+---+-------+
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|18 |John A |
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|18 |Anil B |
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|25 |Zen Hui|
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|25 |Mike A |
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|16 |Shone S|
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|16 |Jack N |
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+---+-------+
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{% endhighlight %}
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### Related Clauses
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- [SELECT Main](sql-ref-syntax-qry-select.html)
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- [WHERE Clause](sql-ref-syntax-qry-select-where.html)
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- [GROUP BY Clause](sql-ref-syntax-qry-select-groupby.html)
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- [HAVING Clause](sql-ref-syntax-qry-select-having.html)
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- [ORDER BY Clause](sql-ref-syntax-qry-select-orderby.html)
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- [SORT BY Clause](sql-ref-syntax-qry-select-sortby.html)
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- [DISTRIBUTE BY Clause](sql-ref-syntax-qry-select-distribute-by.html)
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- [LIMIT Clause](sql-ref-syntax-qry-select-limit.html)
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