Re: [eclipse-clp-users] Dbi Query with more than 1 columns

From: Kish Shen <kisshen_at_...5...>
Date: Thu, 29 Jan 2015 07:48:52 +0000
Hi Shirang,

On 28/01/2015 08:53, Edgaonkar, Shrirang wrote:

> Check the following example which fails since tbl has two columns and first one is a Varchar(cityname).
> Eclipse tool crashes after execution of the script.

What exactly do you mean by "Eclipse tool crashes after execution of the 
script"? I expect the program you show below to abort with a type error 
when it is executed, when cursor_all_tuples/2 is called. Is this what 
you are seeing? If not, please explain exactly what you mean by crash, 
and more precise details on how the crash happened (which precise 
version of ECLiPSe you are running, on what platform, etc.)


>
>   % SQL
>   SQL = "select  c1.cityname, c1.cityid from simulator.city c1",
>
>   % Template
>   Template = tbl(A, 1),

If your table's first column is a string type (VARCHAR), the type 
specifier in the template should be a string, i.e. you should use a 
template like

Template = tbl("london", 1)

For the template tbl(A, 1) you are using, the A -- a variable -- specify 
that you expect the first column to be storing Prolog terms.
Such Prolog terms must have been inserted into the table from dbi, using 
a prepared statement, because a special binary representation is passed 
to MySQL by dbi.

>   session_sql_query(Session, Template, SQL, Cursor),
>
>   % Get Results
>   cursor_all_tuples(Cursor, Tuple),

When dbi reads this data back from SQL in a tuple, as you are doing 
here, it expect the data for the first column to be in this special 
binary format, which it will then convert back into a Prolog term.
If you did not write the data for this column using dbi as a general 
Prolog term, the data will not be in the correct format, and 
cursor_all_tuples/2 should throw a type error.


>
>   Template = tbl(A),   % Success

I am surprised that this would succeed. Please give more details on 
exactly what you did here.

>   Template = tbl(0, 1),   % Success where 0 and 1 are integer fields(not displayed in the above script)

I am not sure what you are doing here exactly. Are you using the same 
query and table, where the first column is of type VARCHAR? I would not 
expect a string to be converted into an integer, unless the string 
happens to be the characters representing a number (e.g. "12"), but 
these are not city names...

> Template = tbl(0),   % Success

Again, what exactly are you doing here? The documentation suggest that 
an error should be thrown if the template has a different arity from the 
tuple.

Cheers,

Kish
Received on Thu Jan 29 2015 - 07:49:11 CET

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