Re: [boost] [relational] Relational Model Library

From: Calum Grant (calum_at_[hidden])
Date: 2005-10-03 15:07:01

> > Also, many queries have a large number of conditions, so the C++ 
> > syntax will get very long and so is *much* harder to read 
> (and write!)
> > 
> > As for avoiding macros, IMHO using macros is fine in this context. 
> > Macros are code-generators, and that's exactly what Calum's 
> using them 
> > for.  Again, there's no reason a declaration of a table in 
> the DSL has 
> > to look like a C++ declaration.
> 
> Macros are useful tools - you only have to look at 
> Boost.PreProcessor to 
> see how powerful it can be! However, macros/preprocessor sould be, 
> wherever possible, restricted to implementation (like Boost.Function).
> 
> As I mentioned above, having a Boost.Spirit-style SQL syntax would 
> complement the direction that C++/Boost is going with respect to 
> describing external constructs (RegExes, BNFL grammars) within C++.
> 
> The RML database and results could be kept in tuples, so CREATE TABLE 
> would be:

No - RML results are iterated not stored.

>     typedef boost::tuple< std::string, std::string, int >
>             people_table;
>     static const int first_name = 0;
>     ...

I think an enum would be more appropriate.

> I only have limited knowledge of SQL syntax, but we could then have 
> something like:
> 
>     rml::sql_statement results =
>        select_( item_<first_name> && item_<last_name> )
>        .where_
>        [
>           item_<age> >= 30
>        ]
> 
>     BOOST_FOREACH( data = results( rml_database ))
>     {
>        std::cout << get<first_name>( data ) << std::endl;
>     }

I think the misconception is that results are not stored in a temporary
table.

The problem with the above is the type of rml::sql_statement.  It is
very complicated and would be different for each query.  Specifically
the iterator is generated to be able to evaluate the query whilst making
use of indexes, and is a different type each time.

You would need

        auto statement_results = select(people, people.age>=30);

which is significantly tidier than the Spiritesque syntax.

Calum