In my table have two columns, one as timestamp to save the date and one as time string to save the time with period.  

Eg:   

[![enter image description here](https://i.stack.imgur.com/GiEEB.png)](https://i.stack.imgur.com/GiEEB.png)

I want to combine them into one column as DateTime format in the result then order by desc on that column.

Here is the example: <http://sqlfiddle.com/#!9/25eb21/4>

The column name 'Datetime' expected is Datetime or timestamps type, so I can sort correctly on it.