>> What kind of SQL statements? Sum? Max? Min? Huh!
>
> SELECT ... FROM ... WHERE FIELDA BETWEEN :X and :Y

Nothing is impossible for real perverts. Let's take set of values of
fieldx, sort it and assign an sequenced integer value to each value.
After that we encrypt fieldx and put it into database along with this
integer value as fielda.
Since then we can find out appropriate integers using encrypted
fieldx and put them into the query for fielda.
For example:

1 Ann
2 Dimitry
3 Sean

select * from ... where name between 'Ann' and 'Sean'
is equivalent to
select * from ... where number between 1 and 3.

SY, SD.