Interesting, I imagine it would have a huge performance problem as it would need to decrypt all the data before running the average but that's understandable.
Do you really need to encrypt the individual values in a database?
I have never heard of someone encrypting the values of every column in a table.
Not every column, certain data that is considered too sensitive - passwords, and certain numeric fields like scores that are very valuable to the client.
There's two main reasons:
So that developers internally don't have access to the clients sensitive data.
If security is breached and someone has access to the data they can't make sense of it.