In SQL and iMIS, the underscore is a wildcard for one character in a LIKE query.  i.e.  select * from orders where product_code like ‘A_’ will return all orders with product codes that are 2 characters in length and begin with an A. 
What if you wanted all orders with a product_code that begins with an A_?  Since the underscore is a recognized wildcard, using select * from orders where product_code like ‘A_%’  will return all orders with product codes that begin with A.  If you want orders that have product codes that begin with A_, place brackets around the underscore: select * from orders where product_code like ’A[_]%’.