Re: [Haskelldb-users] simple question

[Justin B. <jgb...@gm...>]

Just like SQL, HaskellDB has the "isNull" operator. You can use that
to test if a column is null. I've found the best way to use HaskellDB
is to think how I would write a query in SQL, and then find the
equivalents in HaskellDB.

As for the optimized SQL, I agree. There isn't anything in the package
that you aren't seeing. If you see opportunity for improvements please
send a message or better yet, a patch!

On Sat, May 23, 2009 at 1:43 AM, Mathijs Kwik <blu...@gm...> wrote:
> 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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>