libsqlmm is a C++ library that allows you to use several (currently MySQL and PostgreSQL) databases without changing code.

Of course you may have to alter queries.

Example use:

SQL_Database DB = SQL_Connect(SQL_POSTGRESQL, 'db.example.com', 'main_db',
            'www-db', 'secret-pass');
if (!DB.Connected())
    cout << 'Failed to connect to SQL server.' << endl;

SQL_Tuples T = DB.Execute('SELECT * FROM my_table');
if (!T.Success())
    cout << T.GetError().String() << endl;

for (int i = 0; i < T.NumTuples(); ++i)
{
    for (int j = 0; j < T.NumFields(); ++j)
        cout << '| ' << T.GetValue(i, j, 'NULL');
    cout << endl;
}