SQLisp is a functional modeling of the relational algebra in Common Lisp. SQLisp functions return objects that embody the relation expressed by the function call. This allows you to easily integrate your relational expressions with the rest of your Lisp code, instead of requiring the use of a separate language (SQL). It is released under an MIT-style license.
Given an SQLisp object, sqlisp:sql returns a string containing the corresponding SQL. In theory, this allows SQLisp to be used with any database library; in practice, of course, the SQL used by the different RDBMS is different enough that the SQL generation will need to be ported for each RDBMS. SQLisp is currently developed with PostgreSQL, but porting to another reasonable RDBMS should be straightforward.