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### What changes were proposed in this pull request? Document LIMIT clause of SELECT statement in SQL Reference Guide. ### Why are the changes needed? Currently Spark lacks documentation on the supported SQL constructs causing confusion among users who sometimes have to look at the code to understand the usage. This is aimed at addressing this issue. ### Does this PR introduce any user-facing change? Yes. **Before:** There was no documentation for this. **After.** <img width="972" alt="Screen Shot 2020-01-20 at 1 37 28 AM" src="https://user-images.githubusercontent.com/14225158/72715533-7e7a6280-3b25-11ea-98fc-ed68b5d5024a.png"> <img width="972" alt="Screen Shot 2020-01-20 at 1 37 43 AM" src="https://user-images.githubusercontent.com/14225158/72715549-83d7ad00-3b25-11ea-98b3-610eca2628f6.png"> ### How was this patch tested? Tested using jykyll build --serve Closes #27290 from dilipbiswal/sql-ref-select-limit. Authored-by: Dilip Biswal <dkbiswal@gmail.com> Signed-off-by: Sean Owen <srowen@gmail.com>
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| layout | title | displayTitle | license |
|---|---|---|---|
| global | LIMIT Clause | LIMIT 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 LIMIT clause is used to constrain the number of rows returned by the SELECT statement.
In general, this clause is used in conjuction with ORDER BY to ensure that the results are deterministic.
Syntax
{% highlight sql %} LIMIT { ALL | integer_expression } {% endhighlight %}
Parameters
ALL- If specified, the query returns all the rows. In other words, no limit is applied if this option is specified.
integer_expression- Specifies an expression that returns an integer.
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);
-- Select the first two rows. SELECT name, age FROM person ORDER BY name LIMIT 2;
+------+---+ |name |age| +------+---+ |Anil B|18 | |Jack N|16 | +------+---+
-- Specifying ALL option on LIMIT returns all the rows. SELECT name, age FROM person ORDER BY name LIMIT ALL;
+-------+---+ |name |age| +-------+---+ |Anil B |18 | |Jack N |16 | |John A |18 | |Mike A |25 | |Shone S|16 | |Zen Hui|25 | +-------+---+
-- A function expression as an input to limit. SELECT name, age FROM person ORDER BY name LIMIT length('SPARK')
+-------+---+ | name|age| +-------+---+ | Anil B| 18| | Jack N| 16| | John A| 18| | Mike A| 25| |Shone S| 16| +-------+---+ {% endhighlight %}