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### 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>
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| layout | title | displayTitle | license |
|---|---|---|---|
| global | CLUSTER BY Clause | CLUSTER BY Clause | Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements. See the NOTICE file distributed with this work for additional information 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 http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, 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. |
The CLUSTER BY clause is used to first repartition the data based
on the input expressions and then sort the data within each partition. This is
semantically equivalent to performing a
DISTRIBUTE BY followed by a
SORT BY. This clause only ensures that the
resultant rows are sorted within each partition and does not guarantee a total order of output.
Syntax
{% highlight sql %} CLUSTER BY { expression [ , ... ] } {% endhighlight %}
Parameters
expression- Specifies combination of one or more values, operators and SQL functions that results in a value.
Examples
{% highlight sql %} CREATE TABLE person (name STRING, age INT); INSERT INTO person VALUES ('Zen Hui', 25), ('Anil B', 18), ('Shone S', 16), ('Mike A', 25), ('John A', 18), ('Jack N', 16);
-- Reduce the number of shuffle partitions to 2 to illustrate the behavior of CLUSTER BY.
-- It's easier to see the clustering and sorting behavior with less number of partitions.
SET spark.sql.shuffle.partitions = 2;
-- Select the rows with no ordering. Please note that without any sort directive, the results
-- of the query is not deterministic. It's included here to show the difference in behavior
-- of a query when CLUSTER BY is not used vs when it's used. The query below produces rows
-- where age column is not sorted.
SELECT age, name FROM person;
+---+-------+ |age|name | +---+-------+ |16 |Shone S| |25 |Zen Hui| |16 |Jack N | |25 |Mike A | |18 |John A | |18 |Anil B | +---+-------+
-- Produces rows clustered by age. Persons with same age are clustered together. -- In the query below, persons with age 18 and 25 are in first partition and the -- persons with age 16 are in the second partition. The rows are sorted based -- on age within each partition. SELECT age, name FROM person CLUSTER BY age;
+---+-------+ |age|name | +---+-------+ |18 |John A | |18 |Anil B | |25 |Zen Hui| |25 |Mike A | |16 |Shone S| |16 |Jack N | +---+-------+ {% endhighlight %}