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spark-instrumented-optimizer/docs/sql-ref-syntax-qry-sampling.md
Takeshi Yamamuro 179289f0bf [SPARK-31383][SQL][DOC] Clean up the SQL documents in docs/sql-ref* 
### What changes were proposed in this pull request?

This PR intends to clean up the SQL documents in `doc/sql-ref*`.
Main changes are as follows;

 - Fixes wrong syntaxes and capitalize sub-titles
 - Adds some DDL queries in `Examples` so that users can run examples there
 - Makes query output in `Examples` follows the `Dataset.showString` (right-aligned) format
 - Adds/Removes spaces, Indents, or blank lines to follow the format below;

```
---
license...
---

### Description

Writes what's the syntax is.

### Syntax

{% highlight sql %}
SELECT...
    WHERE... // 4 indents after the second line
    ...
{% endhighlight %}

### Parameters

<dl>

  <dt><code><em>Param Name</em></code></dt>
  <dd>
    Param Description
  </dd>
  ...
</dl>

### Examples

{% highlight sql %}
-- It is better that users are able to execute example queries here.
-- So, we prepare test data in the first section if possible.
CREATE TABLE t (key STRING, value DOUBLE);
INSERT INTO t VALUES
    ('a', 1.0), ('a', 2.0), ('b', 3.0), ('c', 4.0);

-- query output has 2 indents and it follows the `Dataset.showString`
-- format (right-aligned).
SELECT * FROM t;
  +---+-----+
  |key|value|
  +---+-----+
  |  a|  1.0|
  |  a|  2.0|
  |  b|  3.0|
  |  c|  4.0|
  +---+-----+

-- Query statements after the second line have 4 indents.
SELECT key, SUM(value)
    FROM t
    GROUP BY key;
  +---+----------+
  |key|sum(value)|
  +---+----------+
  |  c|       4.0|
  |  b|       3.0|
  |  a|       3.0|
  +---+----------+
...
{% endhighlight %}

### Related Statements

 * [XXX](xxx.html)
 * ...
```

### Why are the changes needed?

The most changes of this PR are pretty minor, but I think the consistent formats/rules to write documents are important for long-term maintenance in our community

### Does this PR introduce any user-facing change?

Yes.

### How was this patch tested?

Manually checked.

Closes #28151 from maropu/MakeRightAligned.

Authored-by: Takeshi Yamamuro <yamamuro@apache.org>
Signed-off-by: Sean Owen <srowen@gmail.com>
2020-04-12 23:40:36 -05:00

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layout title displayTitle license
global Sampling Queries Sampling Queries 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.

Description

The TABLESAMPLE statement is used to sample the table. It supports the following sampling methods:

  • TABLESAMPLE(x ROWS): Sample the table down to the given number of rows.
  • TABLESAMPLE(x PERCENT): Sample the table down to the given percentage. Note that percentages are defined as a number between 0 and 100.
  • TABLESAMPLE(BUCKET x OUT OF y): Sample the table down to a x out of y fraction.

Note: TABLESAMPLE returns the approximate number of rows or fraction requested.

Syntax

{% highlight sql %} TABLESAMPLE ((integer_expression | decimal_expression) PERCENT) | TABLESAMPLE (integer_expression ROWS) | TABLESAMPLE (BUCKET integer_expression OUT OF integer_expression) {% endhighlight %}

Examples

{% highlight sql %} SELECT * FROM test; +--+----+ |id|name| +--+----+ | 5|Alex| | 8|Lucy| | 2|Mary| | 4|Fred| | 1|Lisa| | 9|Eric| |10|Adam| | 6|Mark| | 7|Lily| | 3|Evan| +--+----+

SELECT * FROM test TABLESAMPLE (50 PERCENT); +--+----+ |id|name| +--+----+ | 5|Alex| | 2|Mary| | 4|Fred| | 9|Eric| |10|Adam| | 3|Evan| +--+----+

SELECT * FROM test TABLESAMPLE (5 ROWS); +--+----+ |id|name| +--+----+ | 5|Alex| | 8|Lucy| | 2|Mary| | 4|Fred| | 1|Lisa| +--+----+

SELECT * FROM test TABLESAMPLE (BUCKET 4 OUT OF 10); +--+----+ |id|name| +--+----+ | 8|Lucy| | 2|Mary| | 9|Eric| | 6|Mark| +--+----+ {% endhighlight %}

Related Statement