src/Rel8/Query/Distinct.hs · rel8

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rel8/src/Rel8/Query/Distinct.hs

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1{-# options_ghc -fno-warn-redundant-constraints #-}

3module Rel8.Query.Distinct

4 ( distinct

5 , distinctOn

6 , distinctOnBy

7 )

8where

10-- base

11import Prelude

13-- opaleye

14import qualified Opaleye.Distinct as Opaleye

15import qualified Opaleye.Internal.Order as Opaleye

16import qualified Opaleye.Internal.QueryArr as Opaleye

18-- rel8

19import Rel8.Order ( Order( Order ) )

20import Rel8.Query ( Query )

21import Rel8.Query.Opaleye ( mapOpaleye )

22import Rel8.Table.Eq ( EqTable )

23import Rel8.Table.Opaleye ( distinctspec, unpackspec )

26-- | Select all distinct rows from a query, removing duplicates. @distinct q@

27-- is equivalent to the SQL statement @SELECT DISTINCT q@.

29-- >>> select c $ distinct $ values [ lit True, lit True, lit False ]

30-- [False,True]

31distinct :: EqTable a => Query a -> Query a

32distinct = mapOpaleye (Opaleye.distinctExplicit distinctspec)

35-- | Select all distinct rows from a query, where rows are equivalent according

36-- to a projection. If multiple rows have the same projection, it is

37-- unspecified which row will be returned. If this matters, use 'distinctOnBy'.

38distinctOn :: EqTable b => (a -> b) -> Query a -> Query a

39distinctOn proj =

40 mapOpaleye (\q -> Opaleye.productQueryArr (Opaleye.distinctOn unpackspec proj . Opaleye.runSimpleQueryArr q))

43-- | Select all distinct rows from a query, where rows are equivalent according

44-- to a projection. If there are multiple rows with the same projection, the

45-- first row according to the specified 'Order' will be returned.

46distinctOnBy :: EqTable b => (a -> b) -> Order a -> Query a -> Query a

47distinctOnBy proj (Order order) =

48 mapOpaleye (\q -> Opaleye.productQueryArr (Opaleye.distinctOnBy unpackspec proj order . Opaleye.runSimpleQueryArr q))