src/Rel8/Query/Distinct.hs · rel8

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rel8/src/Rel8/Query/Distinct.hs

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1module Rel8.Query.Distinct

2 ( distinct

3 , distinctOn

4 , distinctOnBy

5 )

6where

8-- base

9import Prelude ()

11-- opaleye

12import qualified Opaleye.Distinct as Opaleye

13import qualified Opaleye.Order as Opaleye

15-- rel8

16import Rel8.Order ( Order( Order ) )

17import Rel8.Query ( Query )

18import Rel8.Query.Opaleye ( mapOpaleye )

19import Rel8.Table.Eq ( EqTable )

20import Rel8.Table.Opaleye ( distinctspec, unpackspec )

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

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

25distinct :: EqTable a => Query a -> Query a

26distinct = mapOpaleye (Opaleye.distinctExplicit distinctspec)

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

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

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

32distinctOn :: EqTable b => (a -> b) -> Query a -> Query a

33distinctOn proj = mapOpaleye (Opaleye.distinctOnExplicit unpackspec proj)

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

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

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

39distinctOnBy :: EqTable b => (a -> b) -> Order a -> Query a -> Query a

40distinctOnBy proj (Order order) = mapOpaleye (Opaleye.distinctOnByExplicit unpackspec proj order)