Add support for EXISTS and NOT EXISTS.

src/Database/Esqueleto.hs

@@ -24,6 +24,7 @@ module Database.Esqueleto
              , (==.), (>=.), (>.), (<=.), (<.), (!=.), (&&.), (||.)
              , (+.), (-.), (/.), (*.)
              , like, (%), concat_, (++.)
+             , exists, notExists
              , set, (=.), (+=.), (-=.), (*=.), (/=.) )
   , from
   , Value(..)

src/Database/Esqueleto/Internal/Language.hs

@@ -240,6 +240,21 @@ class (Functor query, Applicative query, Monad query) =>
   -- Supported by SQLite and PostgreSQL.
   (++.) :: (PersistField s, IsString s) => expr (Value s) -> expr (Value s) -> expr (Value s)
 
+  -- | @EXISTS@ operator.  For example:
+  --
+  -- @
+  -- select $
+  -- from $ \person -> do
+  -- where_ $ exists $
+  --          from $ \post -> do
+  --          where_ (post ^. BlogPostAuthorId ==. person ^. PersonId)
+  -- return person
+  -- @
+  exists :: query () -> expr (Value Bool)
+
+  -- | @NOT EXISTS@ operator.
+  notExists :: query () -> expr (Value Bool)
+
   -- | @SET@ clause used on @UPDATE@s.  Note that while it's not
   -- a type error to use this function on a @SELECT@, it will
   -- most certainly result in a runtime error.

src/Database/Esqueleto/Internal/Sql.hs

@@ -324,6 +324,9 @@ instance Esqueleto SqlQuery SqlExpr SqlPersist where
   concat_ = unsafeSqlFunction "CONCAT"
   (++.)   = unsafeSqlBinOp    " || "
 
+  exists    = unsafeSqlFunction     "EXISTS " . existsHelper
+  notExists = unsafeSqlFunction "NOT EXISTS " . existsHelper
+
   set ent upds = Q $ W.tell mempty { sdSetClause = map apply upds }
     where
       apply (ESet f) = SetClause (f ent)
@@ -356,6 +359,12 @@ sub mode query = ERaw Parens $ \conn -> first parens (toRawSql mode conn query)
 fromDBName :: Connection -> DBName -> TLB.Builder
 fromDBName conn = TLB.fromText . escapeName conn
 
+existsHelper :: SqlQuery () -> SqlExpr (Value a)
+existsHelper =
+  ERaw Parens .
+  flip (toRawSql SELECT) .
+  (>> return (val True :: SqlExpr (Value Bool)))
+
 
 ----------------------------------------------------------------------
 

test/Test.hs

@@ -463,6 +463,38 @@ main = do
                                   , Entity p3k p3 { personAge = Just 7 }
                                   , Entity p2k p2 { personAge = Just 0 } ]
 
+    describe "lists of values" $ do
+      it "EXISTS works for subList_select" $
+        run $ do
+          p1k <- insert p1
+          p2k <- insert p2
+          p3k <- insert p3
+          _ <- insert (BlogPost "" p1k)
+          _ <- insert (BlogPost "" p3k)
+          ret <- select $
+                 from $ \p -> do
+                 where_ $ exists $
+                          from $ \bp -> do
+                          where_ (bp ^. BlogPostAuthorId ==. p ^. PersonId)
+                 return p
+          liftIO $ ret `shouldBe` [ Entity p1k p1
+                                  , Entity p3k p3 ]
+
+      it "EXISTS works for subList_select" $
+        run $ do
+          p1k <- insert p1
+          p2k <- insert p2
+          p3k <- insert p3
+          _ <- insert (BlogPost "" p1k)
+          _ <- insert (BlogPost "" p3k)
+          ret <- select $
+                 from $ \p -> do
+                 where_ $ notExists $
+                          from $ \bp -> do
+                          where_ (bp ^. BlogPostAuthorId ==. p ^. PersonId)
+                 return p
+          liftIO $ ret `shouldBe` [ Entity p2k p2 ]
+
 
 ----------------------------------------------------------------------