how to create VIRTUAL column with an if Query in MYSQL

how can i create a virtual cloumn with conditions i have a table with columns 'a', 'b', 'c' 'c' should be virtual with conditions => 1- if 'b' < 16.5 the value should be "1" 2- if 'b' BETWEEN 16.5 AND 18.5 the value should be "2" 3- if 'b' = 25 the value should be "3"