Fixed #26608 -- Added support for window expressions (OVER clause).

Thanks Josh Smeaton, Mariusz Felisiak, Sergey Fedoseev, Simon Charettes,
Adam Chainz/Johnson and Tim Graham for comments and reviews and Jamie
Cockburn for initial patch.

tests/db_functions/test_window.py

@@ -0,0 +1,39 @@
+from django.db.models.functions import Lag, Lead, NthValue, Ntile
+from django.test import SimpleTestCase
+
+
+class ValidationTests(SimpleTestCase):
+    def test_nth_negative_nth_value(self):
+        msg = 'NthValue requires a positive integer as for nth'
+        with self.assertRaisesMessage(ValueError, msg):
+            NthValue(expression='salary', nth=-1)
+
+    def test_nth_null_expression(self):
+        msg = 'NthValue requires a non-null source expression'
+        with self.assertRaisesMessage(ValueError, msg):
+            NthValue(expression=None)
+
+    def test_lag_negative_offset(self):
+        msg = 'Lag requires a positive integer for the offset'
+        with self.assertRaisesMessage(ValueError, msg):
+            Lag(expression='salary', offset=-1)
+
+    def test_lead_negative_offset(self):
+        msg = 'Lead requires a positive integer for the offset'
+        with self.assertRaisesMessage(ValueError, msg):
+            Lead(expression='salary', offset=-1)
+
+    def test_null_source_lead(self):
+        msg = 'Lead requires a non-null source expression'
+        with self.assertRaisesMessage(ValueError, msg):
+            Lead(expression=None)
+
+    def test_null_source_lag(self):
+        msg = 'Lag requires a non-null source expression'
+        with self.assertRaisesMessage(ValueError, msg):
+            Lag(expression=None)
+
+    def test_negative_num_buckets_ntile(self):
+        msg = 'num_buckets must be greater than 0'
+        with self.assertRaisesMessage(ValueError, msg):
+            Ntile(num_buckets=-1)