[SPARK-28797][DOC] Document DROP FUNCTION statement in SQL Reference.

docs/sql-ref-syntax-ddl-drop-function.md

@@ -19,4 +19,70 @@ license: |
   limitations under the License.
 ---
 
-**This page is under construction**
+### Description
+Dropping a user-defined function(UDF). An exception will be thrown if the function does not exist
+ in the system. 
+
+### Syntax
+{% highlight sql %}
+DROP [TEMPORARY] FUNCTION [IF EXISTS] [db_name.]function_name;
+{% endhighlight %}
+
+
+### Parameters
+
+<dl>
+  <dt><code><em>function_name</em></code></dt>
+  <dd>The name of an existing function.</dd>
+</dl>
+
+<dl>
+  <dt><code><em>TEMPORARY</em></code></dt>
+  <dd>Should be used to delete the `temporary` function.</dd>
+</dl>
+
+<dl>
+  <dt><code><em>IF EXISTS</em></code></dt>
+  <dd>If specified, no exception is thrown when the function does not exist.</dd>
+</dl>
+
+### Example
+{% highlight sql %}
+-- Create a permanent function `test_avg`
+
+CREATE FUNCTION test_avg as 'org.apache.hadoop.hive.ql.udf.generic.GenericUDAFAverage';
+
+-- List user functions 
+
+SHOW user functions;
+
++-------------------+
+|     function      |
++-------------------+
+| default.test_avg  |
++-------------------+
+
+-- Drop Permanent function
+
+DROP FUNCTION test_avg;
++---------+
+| Result  |
++---------+
++---------+
+
+-- Creaet Temporary function `temp_avg`
+CREATE TEMPORARY FUNCTION temp_avg as 'org.apache.hadoop.hive.ql.udf.generic.GenericUDAFAverage';
+
+-- Drop Temporary function
+
+DROP TEMPORARY FUNCTION IF EXISTS temp_avg;
++---------+
+| Result  |
++---------+
++---------+
+
+{% endhighlight %}
+### Related statements
+- [CREATE FUNCTION](sql-ref-syntax-ddl-create-function.html)
+- [DESCRIBE FUNCTION](sql-ref-syntax-aux-describe-function.html)
+- [SHOW FUNCTION](sql-ref-syntax-aux-show-functions.html)