I am using pandas to add additional columns in the data frame based on the conditions and comparison of values in existing columns.

This is the original data frame:

```
        start         end Sold
0                           NA
1  2017-05-08  2017-09-08  Yes
2  2018-09-01  2017-09-01  Yes

```

This is the data frame that I want:

```
        start         end Sold    valid
0                           NA  Unknown
1  2017-05-08  2017-09-08  Yes     True
2  2018-09-01  2017-09-01  Yes    False

```

Basically, the new column valid is defined by all 3 existing columns.

`Condition 1`: If sold is NA, valid equals to unknown. 

`Condition 2`: If sold is not NA, start date < end date, valid equals to True. And valid equals to False is sold is not NA and start date > end date.

Can anyone suggest a piece of code?