[SPARK-28796][DOC] Document DROP DATABASE statement in SQL Reference

### What changes were proposed in this pull request?
Document DROP DATABASE statement in SQL Reference

### Why are the changes needed?
Currently from spark there is no complete sql guide is present, so it is better to document all the sql commands, this jira is sub part of this task.

### Does this PR introduce any user-facing change?
Yes, Before there was no documentation about drop database syntax

After Fix
![image](https://user-images.githubusercontent.com/35216143/64787097-977a7200-d58d-11e9-911c-d2ff6f3ccff5.png)
![image](https://user-images.githubusercontent.com/35216143/64787122-a6612480-d58d-11e9-978c-9455baff007f.png)

### How was this patch tested?
tested with jenkyll build

Closes apache#25554 from sandeep-katta/dropDbDoc.

Authored-by: sandeep katta <sandeep.katta2007@gmail.com>
Signed-off-by: Xiao Li <gatorsmile@gmail.com>

Author: sandeep-katta

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

@@ -19,4 +19,62 @@ license: |
   limitations under the License.
 ---
 
-**This page is under construction**
+### Description
+
+Drop a database and delete the directory associated with the database from the file system. An 
+exception will be thrown if the database does not exist in the system. 
+
+### Syntax
+
+{% highlight sql %}
+DROP (DATABASE|SCHEMA) [IF EXISTS] dbname [RESTRICT|CASCADE];
+{% endhighlight %}
+
+
+### Parameters
+
+<dl>
+  <dt><code><em>DATABASE|SCHEMA</em></code></dt>
+  <dd>`DATABASE` and `SCHEMA` mean the same thing, either of them can be used.</dd>
+</dl>
+
+<dl>
+  <dt><code><em>IF EXISTS</em></code></dt>
+  <dd>If specified, no exception is thrown when the database does not exist.</dd>
+</dl>
+
+<dl>
+  <dt><code><em>RESTRICT</em></code></dt>
+  <dd>If specified, will restrict dropping a non-empty database and is enabled by default.</dd>
+</dl>
+
+<dl>
+  <dt><code><em>CASCADE</em></code></dt>
+  <dd>If specified, will drop all the associated tables and functions.</dd>
+</dl>
+
+### Example
+{% highlight sql %}
+-- Create `inventory_db` Database
+CREATE DATABASE inventory_db COMMENT 'This database is used to maintain Inventory';
+
+-- Drop the database and it's tables
+DROP DATABASE inventory_db CASCADE;
++---------+
+| Result  |
++---------+
++---------+
+
+-- Drop the database using IF EXISTS
+DROP DATABASE IF EXISTS inventory_db CASCADE;
++---------+
+| Result  |
++---------+
++---------+
+
+{% endhighlight %}
+
+### Related statements
+- [CREATE DATABASE](sql-ref-syntax-ddl-create-database.html)
+- [DESCRIBE DATABASE](sql-ref-syntax-aux-describe-database.html)
+- [SHOW DATABASES](sql-ref-syntax-aux-show-databases.html)