Re: [Haskelldb-users] simple question

[Mathijs K. <blu...@gm...>]

Hi,

Thanks for the quick reply.
I was affraid that fromNull thing was gonna be needed indeed.
While this works in this case, I'm not very happy with it.
NULL != 0, and there might be cases where all possible values are
allowed in a certain column (including 0).
Isn't it possible to do this the other way around? (tell the other,
mandatory field to become Just ...)

Also, while I don't mind yet (since I like the idea of haskelldb very
much), the so-called optimized sql is horrible :)
Maybe some optimizations (things like using inner joins) should be
turned on? How can I do this?

Thanks again,
Mathijs




On Sat, May 23, 2009 at 6:37 AM, Justin Bailey <jgb...@gm...> wrote:
> The error is telling you that you can't compare an "Int" to a "Maybe
> Int", which makes sense becuse one column can hold NULL or an int,
> while the other can only hold ints. Use the fromNull operator.
> Assuming group_id is the column which can be null, I think you want:
>
>  let q = do { u <- table users; g <- table groups; restrict (g !
> G.xid .==. fromNull (constant 0) (u ! U.group_id))}
>
> Justin
>
> On Fri, May 22, 2009 at 6:36 PM, Mathijs Kwik <blu...@gm...> wrote:
>> Hi all,
>>
>> I finally got haskelldb working. It took some time, but (after many
>> tries in the past) I got it.
>> I'm playing around with querying and I have a simple question:
>>
>> I have a table with a foreign key that can be NULL.
>> Now if I try to create an inner-join like restriction, I get a
>> type-mismatch, which I can understand, but couldn't figure out how to
>> solve it (it's a little late here :)
>>
>>  let q = do { u <- table users; g <- table groups; restrict (g!G.xid
>> .==. u!U.group_id)}
>>
>> <interactive>:1:4:
>>    Couldn't match expected type `Int'
>>           against inferred type `Maybe Int'
>>      Expected type: Expr Int
>>      Inferred type: Expr (Maybe Int)
>>    When using functional dependencies to combine
>>      Select (Attr f a) (Rel r) (Expr a),
>>        arising from the dependency `f r -> a'
>>        in the instance declaration at <no location info>
>>      Select (Attr U.Group_id (Maybe Int)) (Rel U.Users) (Expr Int),
>>        arising from a use of `!' at <interactive>:1:97-107
>>    When generalising the type(s) for `q'
>>
>> What would be the correct way to fix this?
>>
>> Thanks,
>> Mathijs
>>
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