Use aggregate functions, like counting rows

Use functions like
count :: Expr a -> Expr Int _sum, _max, avg :: Num a => Expr a -> Expr a

Aggregate functions can only be used in a project. We automatically group on all other fields in the project.
You need a field of the return type to put the value into in your project
aggrSum = withDB $ \ db -> do res <- query db $ do farms <- table farms project (H.farm_id << _sum (fromNull (constant 0) (farms!chickens) )) putStrLn "Number of chickens on all farms:" mapM (putStrLn.showKO) res where showKO r = show (r!H.farm_id) Frame> aggrSum Number of chickens on all farms: 221

If you need for example a field with an Int value but don't have one available, you can use a hack to create one.


 
module NewFieldDef where

import Database.HaskellDB.DBLayout 

data NewField = NewField
instance FieldTag NewField where fieldName _ = "newField"

newTbl :: Table 
    ( RecCons NewField (Expr Int) RecNil)

newTbl =  baseTable "newTbl" $ 
	  hdbMakeEntry NewField

newField :: Attr NewField Int
newField = mkAttr NewField

Then you can import NewFieldDef and use newField as an int field.

Maybe you get an error like this one: aggr = withDB $ \ db -> do res <- query db $ do t <- table H.farms project (H.chickens << _sum (t!H.chickens)) return res Bad2> aggr ERROR "Bad2.hs":6 - Unresolved top-level overloading *** Binding : aggr *** Outstanding context : Num (Maybe Int) ERROR - Undefined variable "aggr"

Then you have to use fromNull to remove the maybe, and put the result into a field wich is not null.

Here are all words and functions with references, in alphabetical order

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