I have two tables, `internal_mail` and `external_mail`, they relatively have different columns. I'm trying to select all records from both tables with ordering by a date column, called `date_of_arrival` in both tables.

For example this is `internal_mail`: 

```
colunm 1          colunm 2          colunm 3          date_of_arrival
some_record       some_record       some_record       2017/8/12
.....             .....             .....             ..... 
.....             .....             .....             ..... 
.....             .....             .....             ..... 
.....             .....             .....             ..... 

```

And this is `external_mail`:

```
colunm A          colunm B          colunm C          date_of_arrival
another_record    another_record    another_record       2017/7/12
.....             .....             .....             ..... 
.....             .....             .....             ..... 
.....             .....             .....             ..... 
.....             .....             .....             ..... 

```

What I'm trying to achieve here is something like this(in case of a descending order by):

```
colunm 1  colunm 2  colunm 3  column A  column B  column C   date_of_arrival
s_r       s_r       s_r       empty     empty     empty      2017/8/12
empty     empty     empty     a_r       a_r       a_r        2017/7/12
s_r       s_r       s_r       empty     empty     empty      2017/8/12
empty     empty     empty     a_r       a_r       a_r        2017/7/12

```