resolve select to NULLTYPE if no columns

Fixed regression where the :func:`_sql.select` construct would not be able
to render if it were given no columns and then used in the context of an
EXISTS, raising an internal exception instead. While an empty "SELECT" is
not typically valid SQL, in the context of EXISTS databases such as
PostgreSQL allow it, and in any case the condition now no longer raises
an internal exception.

For this case, also add an extra whitespace trim step for the unusual
case that there are no columns to render.  This is done in such a
way as to not interfere with other test cases that are involving
custom compilation schemes.

Fixes: #9440
Change-Id: If65ba9ce15d371f09b4342ad0669143b7b082a78

1 files changed, 24 insertions, 0 deletions

diff --git a/test/sql/test_select.py b/test/sql/test_select.py
index ad4b4db95..7979fd200 100644
--- a/test/sql/test_select.py
+++ b/test/sql/test_select.py
@@ -9,6 +9,7 @@ from sqlalchemy import select
 from sqlalchemy import String
 from sqlalchemy import Table
 from sqlalchemy import testing
+from sqlalchemy import true
 from sqlalchemy import tuple_
 from sqlalchemy import union
 from sqlalchemy.sql import column
@@ -77,6 +78,29 @@ class SelectTest(fixtures.TestBase, AssertsCompiledSQL):
             "WHERE mytable.myid = myothertable.otherid",
         )
 
+    @testing.combinations(
+        (
+            lambda tbl: select().select_from(tbl).where(tbl.c.id == 123),
+            "SELECT FROM tbl WHERE tbl.id = :id_1",
+        ),
+        (lambda tbl: select().where(true()), "SELECT WHERE 1 = 1"),
+        (
+            lambda tbl: select()
+            .select_from(tbl)
+            .where(tbl.c.id == 123)
+            .exists(),
+            "EXISTS (SELECT FROM tbl WHERE tbl.id = :id_1)",
+        ),
+    )
+    def test_select_no_columns(self, stmt, expected):
+        """test #9440"""
+
+        tbl = table("tbl", column("id"))
+
+        stmt = testing.resolve_lambda(stmt, tbl=tbl)
+
+        self.assert_compile(stmt, expected)
+
     def test_new_calling_style_clauseelement_thing_that_has_iter(self):
         class Thing:
             def __clause_element__(self):