Jeremy Shaw wrote:
> Hello,
>
> I am not quite sure what the best way to do the following in
> haskelldb is:
>
> Here is the query in SQL:
>
> select column1 from table1 where column2 = (select max(column2) from
> table1);
>
> Basically, I want to find the value in column1 that is in the same
> row as the maximum column2 value. In this case, column2 is a unique
> key.
>
> I could do this as two seperate queries haskelldb, one to find the
> max column2 value, and a second query to find the corresponding
> column1 value -- but it would be nice to do it all in one step. I
> imagine I might also be able to hack something up by using sort and
> top -- but that seems a bit hackish when the actual SQL I want to
> generate is pretty straight forward.
>
> Any ideas?
One way could be to join the table with itself (the most efficient thing
in the world, though):
do
t <- table table1
t' <- table table1
r <- project (column2 << _max (t'!column2))
restrict (t!column2 .==. r!column2)
project (column1 << t!column1)
I think being able to use queries in expressions is a todo item left over
from the original HaskellDB.
It would be nice if something like this would work:
do
t <- table table1
let m = do
t' <- table table1
project (column2 << _max (t'!column2))
restrict (column2 `_in` m)
project (column1 << t!column1)
That is, we would have a function:
_in :: Eq a => Expr a -> Query (Rel (RecCons f a RecNil)) -> Expr Bool
That doesn't actually work, since the result should be in the Query monad,
but something along these lines should be doable.
/Bjorn
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