Whats the trick for creating a parameterised view with arbitrary filter conditions (held - eg - in a string variable called "qrycode") ?

I've tried

SELECT Contacts.*;
FROM ;
CONTACT!CONTACTS;
WHERE (?qrycode)

which crashes either with "function name missing" or "invalid sql clause"

and

WHERE EVALUATE(?qrycode)

which doesn't crash, but never finds any records, even when I set "qrycode" for known matches.

The idea is to allow users to create their own queries as parameterised views on whatever fields and conditions suit them at the time.