huaxingao commented on a change in pull request #27283: [SPARK-30574][DOC]
Document GROUP BY Clause of SELECT statement in SQL Reference.
URL: https://github.com/apache/spark/pull/27283#discussion_r368360778
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File path: docs/sql-ref-syntax-qry-select-groupby.md
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@@ -18,5 +18,206 @@ license: |
See the License for the specific language governing permissions and
limitations under the License.
---
+The <code>GROUP BY</code> clause is used to group the rows based on a set of
specified grouping expressions and compute aggregations on
+the group of rows based on one or more specified aggregate functions. Spark
also supports advanced aggregations to do multiple
+aggregations for the same input record set via `GROUPING SETS`, `CUBE`,
`ROLLUP` clauses.
-**This page is under construction**
+### Syntax
+{% highlight sql %}
+GROUP BY [ GROUPING SETS grouping_sets ] group_expression [ , group_expression
[ , ... ] ]
+ [ ( WITH ROLLUP | WITH CUBE | GROUPING SETS grouping_sets ) ) ]
+{% endhighlight %}
+
+### Parameters
+<dl>
+ <dt><code><em>GROUPING SETS</em></code></dt>
+ <dd>
+ Groups the rows for each subset of the expressions specified in the
grouping sets. For example,
+ <code>GROUP BY GROUPING SETS (warehouse, product)</code> is semantically
equivalent
+ to union of results of <code>GROUP BY warehouse</code> and <code>GROUP BY
product</code>. This clause
+ is a short-hand to using an <code>UNION ALL</code> where each leg of the
<code>UNION ALL</code>
+ operator performs aggregation of subset of the columns specified in the
<code>GROUPING SETS</code> clause.
+ </dd>
+ <dt><code><em>grouping_sets</em></code></dt>
+ <dd>
+ Specifies one of more groupings based on which the <code>GROUP BY</code>
clause performs aggregations. A grouping
+ set is specified by a list of comma separated expressions in
parenthses.<br><br>
+ <b>Syntax:</b>
+ <code>
+ (() | (expression [ , ...]))
+ </code>
+ </dd>
+ <dt><code><em>grouping_expression</em></code></dt>
+ <dd>
+ Specifies the crieteria based on which the rows are grouped together. A
grouping expression may be a column alias,
+ a column position or an expression.
+ </dd>
+ <dt><code><em>ROLLUP</em></code></dt>
+ <dd>
+ Specifies multiple levels of aggregations in a single statement. This
clause is used to compute aggregations
+ based on multiple grouping sets. <code>ROLLUP</code> is a short-hand for
<code>GROUPING SETS</code>. For example,
+ GROUP BY warehouse, product WITH ROLLUP is equivalent to GROUP BY
warehouse, product GROUPING SETS ((warehouse, product), (warehouse), ()).
+ The N elements of a <code>ROLLUP</code> specification results in N+1
<code>GROUPING SETS</code>.
+ </dd>
+ <dt><code><em>CUBE</em></code></dt>
+ <dd>
+ <code>CUBE</code> clause is used to perform aggregations based on
combination of grouping columns specified in the
+ <code>GROUP BY</code> clause. For example, <code>GROUP BY warehouse,
product WITH CUBE</code> is equivalent
+ to GROUP BY warehouse, product GROUPING SETS ((warehouse, product),
(warehouse), (product), ()).
+ The N elements of a <code>CUBE</code> specification results in 2^N
<code>GROUPING SETS</code>.
+ </dd>
+</dl>
+
+### Examples
+{% highlight sql %}
+CREATE TABLE dealer (id INT, city STRING, car_model STRING, quantity INT);
+INSERT INTO dealer VALUES (100, 'Fremont', 'Honda Civic', 10),
Review comment:
There is an extra space between ```dealer``` and ```VALUES```
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