I have given an assignment to get the feasibility for making a dll which takes some inputs such as database name(Mysql,access, sql, oracle etc.) and some more inputs to generate a query and based on that the dll should return a recordset to the application.
is it a good idea. if yes then what inputs should i consider
It's completely feasible, the issues arise when you encounter different SQL grammars - take a look at how Hibernate handles this with the use of Dialects.
The popular databases - Oracle, Sybase, MS SQL Server, MySQL - have slight differences in the SQL grammar they allow. Essentually the vendors have implemented and extended ANSI SQL in different ways.
The simplest case I can think of is when the way you assign pseudonyms to column names, some databases require this:
SELECT x AS y FROM some_table
while others require:
SELECT x y FROM some_table
There's many more such examples, but the bottom line is that when writing a query abstraction layer that works across all databases you need to abstract the concept of SQL generation so that it can be tailored to each database you are going to support (as I said, Hibernate does this by allowing you to specify a dialect specific to the database you are using).