I am trying to populate blank cell with a value from another column, i.e.
if 'country' is blank then populate with a value from 'country_1'
I tried the below, but it doesn't seem to work.
IFNULL (`country`, `country_1`)
The previously proposed formula will fix your issue, however if you need to use case statement (for another reason) the following should work I think
WHEN TRIM(`Country`) = ''
might want to use a case statement for blanks and nulls.
Case when trim(`country`) = '' or `country` is null then `country` else `country` end