Problem Passing SQL Via Querystring

My problem is I'm paginating database results into a table, and each page takes the sql statement from the querystring, however because i use LIKE %...% the '%' get ignored and I get an error. For example the sql in the querystring is..

SELECT * FROM posters WHERE poster_name LIKE '%a%' AND poster_file IS NOT NULL ORDER BY poster_name ASC;

But when it's extracted from the querystring it is this:

SELECT * FROM posters WHERE poster_name LIKE 'a' AND poster_file IS NOT NULL ORDER BY poster_name ASC;

So i get an error because the wildcards have been removed. Any way around this?