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

2 files changed, 13 insertions, 0 deletions

diff --git a/lib/sqlalchemy/sql/compiler.py b/lib/sqlalchemy/sql/compiler.py
index bc463f9a1..ad0a3b686 100644
--- a/lib/sqlalchemy/sql/compiler.py
+++ b/lib/sqlalchemy/sql/compiler.py
@@ -4668,6 +4668,16 @@ class SQLCompiler(Compiled):
             from_linter = None
             warn_linting = False
 
+        # adjust the whitespace for no inner columns, part of #9440,
+        # so that a no-col SELECT comes out as "SELECT WHERE..." or
+        # "SELECT FROM ...".
+        # while it would be better to have built the SELECT starting string
+        # without trailing whitespace first, then add whitespace only if inner
+        # cols were present, this breaks compatibility with various custom
+        # compilation schemes that are currently being tested.
+        if not inner_columns:
+            text = text.rstrip()
+
         if froms:
             text += " \nFROM "
 
diff --git a/lib/sqlalchemy/sql/selectable.py b/lib/sqlalchemy/sql/selectable.py
index 39ef420dd..56cca6f73 100644
--- a/lib/sqlalchemy/sql/selectable.py
+++ b/lib/sqlalchemy/sql/selectable.py
@@ -89,6 +89,7 @@ from .elements import literal_column
 from .elements import TableValuedColumn
 from .elements import UnaryExpression
 from .operators import OperatorType
+from .sqltypes import NULLTYPE
 from .visitors import _TraverseInternalsType
 from .visitors import InternalTraversal
 from .visitors import prefix_anon_map
@@ -5167,6 +5168,8 @@ class Select(
         GenerativeSelect.__init__(self)
 
     def _scalar_type(self) -> TypeEngine[Any]:
+        if not self._raw_columns:
+            return NULLTYPE
         elem = self._raw_columns[0]
         cols = list(elem._select_iterable)
         return cols[0].type