Thread: [Haskelldb-users] Cartesian product

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How is it possible to do join between tables?

Tom

Thomas L. Bevan wrote:
> How is it possible to do join between tables?

Hi Tom,

here is an example of a join between two tables:

       do
       r <- table time_reports
       u <- table users
       restrict (r!userid .==. u!xid)
       project (first_name << u!first_name
                # last_name << u!last_name
                # activity << r!activity)

/Bjorn

Thanks. I had a problem with the code below. It compiles but generates a 
runtime exception.

Fail: user error (SqlError {seState = "E", seNativeError = 7, seErrorMsg = 
"ERROR:  column \"username1\" does not exist\n"})

userIds = do
 users <- table phpbb_users
 group <- table phpbb_user_group
 -- restrict (users!(U.user_id) .==. group!(G.user_id) )
 project ( username << users!username #
           user_email << users!user_email #
           user_regdate << users!user_regdate #
           group_id << group!group_id )

If I uncomment the restriction everything runs smoothly. I suspect that there 
is a lacunae in the SQL generation code for cases where the JOIN is 
unqualified.

Tom

On Tue, 16 Mar 2004 07:19 pm, Bjorn Bringert wrote:
> Thomas L. Bevan wrote:
> > How is it possible to do join between tables?
>
> Hi Tom,
>
> here is an example of a join between two tables:
>
>        do
>        r <- table time_reports
>        u <- table users
>        restrict (r!userid .==. u!xid)
>        project (first_name << u!first_name
>                 # last_name << u!last_name
>                 # activity << r!activity)
>
> /Bjorn
>
>
>
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A poet who reads his verse in public may have other nasty habits.

Thomas L. Bevan wrote:
> Thanks. I had a problem with the code below. It compiles but generates a 
> runtime exception.
> 
> Fail: user error (SqlError {seState = "E", seNativeError = 7, seErrorMsg = 
> "ERROR:  column \"username1\" does not exist\n"})
> 
> userIds = do
>  users <- table phpbb_users
>  group <- table phpbb_user_group
>  -- restrict (users!(U.user_id) .==. group!(G.user_id) )
>  project ( username << users!username #
>            user_email << users!user_email #
>            user_regdate << users!user_regdate #
>            group_id << group!group_id )
> 
> If I uncomment the restriction everything runs smoothly. I suspect that there 
> is a lacunae in the SQL generation code for cases where the JOIN is 
> unqualified.

Thanks for reporting this, it sure looks like a bug. I get the same 
thing with other joins without a restrict too, never tried that before. 
We'll look into it.

/Bjorn