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

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

@@ -19,4 +19,87 @@ license: |
   limitations under the License.
 ---
 
-**This page is under construction**
+### Description
+The `DROP FUNCTION` statement drops a temporary or user defined function (UDF). An exception will
+ be thrown if the function does not exist. 
+
+### 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  |
+  +-------------------+
+
+-- Create Temporary function `test_avg`
+CREATE TEMPORARY FUNCTION test_avg as 'org.apache.hadoop.hive.ql.udf.generic.GenericUDAFAverage';
+
+-- List user functions
+SHOW USER FUNCTIONS;
+  +-------------------+
+  |     function      |
+  +-------------------+
+  | default.test_avg  |
+  | test_avg          |
+  +-------------------+
+
+-- Drop Permanent function
+DROP FUNCTION test_avg;
+  +---------+
+  | Result  |
+  +---------+
+  +---------+
+
+-- Try to drop Permanent function which is not present
+DROP FUNCTION test_avg;
+  Error: Error running query:
+  org.apache.spark.sql.catalyst.analysis.NoSuchPermanentFunctionException:
+  Function 'default.test_avg' not found in database 'default'; (state=,code=0)
+
+-- List the functions after dropping, it should list only temporary function
+SHOW USER FUNCTIONS;
+  +-----------+
+  | function  |
+  +-----------+
+  | test_avg  |
+  +-----------+
+
+-- Drop Temporary function
+DROP TEMPORARY FUNCTION IF EXISTS test_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)