Re: [eclipse-clp-users] database interface

From: Kish Shen <kisshen_at_...5...>
Date: Thu, 30 Oct 2008 15:50:57 +0000
Hi Carlo,

I am posting this to the mailing list again as it may be of more general 
interest.

You are right. I should have looked at the dbi code more closely, as I 
have not looked at it for a while now.

The way to get tuples on backtracking is to do it like 
session_retrieve_tuple/4 in dbi.ecl, using a repeat:

student(Cursor, Age) :-
	repeat,
	(cursor_next_tuple(Cursor,Age) -> true ; !, fail).


I agree that the manual is not too clear on this. I will try and improve 
that.

Thanks and cheers,

Kish

Carlo Matteo Scalzo wrote:
> Here is the code: i'm assuming i have already created a mysql database
> called 'dbpl', and a user called 'dbpluser' (password: dbpluser) who has the
> right to access the dbpl database.
> 
> The first call creates a table 'students' having a single column ('age'),
> and adds 3 rows to it:
> 
> 
> session_start(dbpluser,dbpluser,_,Session),
> session_sql(Session,"use dbpl",_),
> session_sql(Session, "create table students (age decimal(2))", _),
> session_sql(Session, "insert into students values (23)", _),
> session_sql(Session, "insert into students values (31)", _),
> session_sql(Session, "insert into students values (16)", _),
> session_commit(Session),
> session_close(Session).
> 
> 
> The second call 'manually' calls cursor_next_tuple/2 (on the query "select *
> from students") and prints the results. It is called 3 times in order to
> retrieve the 3 rows just inserted.
> 
> 
> session_start(dbpluser,dbpluser,_,Session),
> session_sql(Session,"use dbpl",_),
> session_sql_query(Session,students(age),"select * from students", Cursor),
> cursor_next_tuple(Cursor, students(Age1)),
> writeln(Age1),
> cursor_next_tuple(Cursor, students(Age2)),
> writeln(Age2),
> cursor_next_tuple(Cursor, students(Age3)),
> writeln(Age3),
> session_close(Session).
> 
> Calling the above conjunctive goal correctly prints:
> 
> 23
> 31
> 16
> 
> 
> Now the real test. The following code tries to use backtracking on
> cursor_next_tuple/2.
> 
> 
> session_start(dbpluser,dbpluser,_,Session),
> session_sql(Session,"use dbpl",_),
> session_sql_query(Session,students(age),"select * from students", Cursor),
> cursor_next_tuple(Cursor, students(Age)),
> writeln(Age),
> fail.
> 
> 
> Calling the above goal prints:
> 
> 23
> 
> In fact, using the tracer, you can see that cursor_next_tuple/2 is skipped
> on backtracking. It returns the first tuple only.
> 
> 
> Thanks again for your help.
> 


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Received on Thu Oct 30 2008 - 16:24:31 CET

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