Mapping values in SQL select?

You can use case expression:

select
    id,
    name,
    case 
        when category = 'fruits' then 'Fruit Plant'
        when category = 'vegetables' then 'Vegetable Plant'
        when category is null then 'unknown'
    end as category
from Plant

use case function (with else statement for default values) :

select id, name, 
case category
   when 'vegetables' then 'Vegetable Plant'
   when 'fruits' then 'Fruit Plant'
   when is null then 'unknown'
   else 'default values'
end
from Plant