Just got this answer from a previous question and it works a treat!
SELECT username, (SUM(rating)/COUNT(*)) as TheAverage, Count(*) as TheCount
FROM ratings
I also got same error, but in my case main problem was in where condition the parameter that i'm checking was having some unknown hidden character (+%A0)
When A0 convert I got 160 but 160 was out of the range of the character that db knows, that's why database cannot recognize it as character other thing is my table column is varchar
the solution that I did was I checked there is some characters like that and remove those before run the sql command
ex:- preg_replace('/\D/', '', $myParameter);