Fixed #32126 -- Fixed grouping by Case() annotation without cases.

Co-authored-by: Simon Charette <charettes@users.noreply.github.com>

django/db/models/expressions.py

@@ -1068,6 +1068,11 @@ class Case(Expression):
             sql = connection.ops.unification_cast_sql(self.output_field) % sql
         return sql, sql_params
 
+    def get_group_by_cols(self, alias=None):
+        if not self.cases:
+            return self.default.get_group_by_cols(alias)
+        return super().get_group_by_cols(alias)
+
 
 class Subquery(Expression):
     """

tests/expressions_case/tests.py

@@ -1148,6 +1148,31 @@ class CaseExpressionTests(TestCase):
             lambda x: x[1:]
         )
 
+    def test_aggregation_empty_cases(self):
+        tests = [
+            # Empty cases and default.
+            (Case(output_field=IntegerField()), None),
+            # Empty cases and a constant default.
+            (Case(default=Value('empty')), 'empty'),
+            # Empty cases and column in the default.
+            (Case(default=F('url')), ''),
+        ]
+        for case, value in tests:
+            with self.subTest(case=case):
+                self.assertQuerysetEqual(
+                    CaseTestModel.objects.values('string').annotate(
+                        case=case,
+                        integer_sum=Sum('integer'),
+                    ).order_by('string'),
+                    [
+                        ('1', value, 1),
+                        ('2', value, 4),
+                        ('3', value, 9),
+                        ('4', value, 4),
+                    ],
+                    transform=itemgetter('string', 'case', 'integer_sum'),
+                )
+
 
 class CaseDocumentationExamples(TestCase):
     @classmethod