Refs #28643 -- Added NullIf database function.

Thanks Nick Pope, Mariusz Felisiak, and Tim Graham for reviews.

tests/db_functions/comparison/test_nullif.py

@@ -0,0 +1,40 @@
+from unittest import skipUnless
+
+from django.db import connection
+from django.db.models import Value
+from django.db.models.functions import NullIf
+from django.test import TestCase
+
+from ..models import Author
+
+
+class NullIfTests(TestCase):
+
+    @classmethod
+    def setUpTestData(cls):
+        Author.objects.create(name='John Smith', alias='smithj')
+        Author.objects.create(name='Rhonda', alias='Rhonda')
+
+    def test_basic(self):
+        authors = Author.objects.annotate(nullif=NullIf('alias', 'name')).values_list('nullif')
+        self.assertSequenceEqual(
+            authors, [
+                ('smithj',),
+                ('' if connection.features.interprets_empty_strings_as_nulls else None,)
+            ]
+        )
+
+    def test_null_argument(self):
+        authors = Author.objects.annotate(nullif=NullIf('name', Value(None))).values_list('nullif')
+        self.assertSequenceEqual(authors, [('John Smith',), ('Rhonda',)])
+
+    def test_too_few_args(self):
+        msg = "'NullIf' takes exactly 2 arguments (1 given)"
+        with self.assertRaisesMessage(TypeError, msg):
+            NullIf('name')
+
+    @skipUnless(connection.vendor == 'oracle', 'Oracle specific test for NULL-literal')
+    def test_null_literal(self):
+        msg = 'Oracle does not allow Value(None) for expression1.'
+        with self.assertRaisesMessage(ValueError, msg):
+            list(Author.objects.annotate(nullif=NullIf(Value(None), 'name')).values_list('nullif'))