Design question: How to convert a given data structure to sql and the opposite.

Hi,

Configuration:
I'm using postgresql 7.3 on Linux box ( RH 7.3) from Perl DBI module.

The problem:
I'm implementing a very interesting system where i need to enable via user
interface (WEB GUI ) to build/update and execute SQL queries on my DB
scheme.

1. I need to translate a given data structure to SQL query .
2. I need to load SQL query to memory.

Generaly i need to convert from a given data structure to sql and the
opposite which means i want to represent a generic SQL in terms and i want
to be able to
load saved query and execute it.

Just to make things clear i will detail to scenarios:

1 . A User => Creates a query using GUI => he/she can execute the query
2. A query is stored somehow => The user update the query according to his
needs => the user save the query and then execute it.

Any idea/direction will be welcome ?

Hanan