implementation for <aggregate_fun> FILTER (WHERE ...)

1 files changed, 23 insertions, 1 deletions

diff --git a/lib/sqlalchemy/sql/functions.py b/lib/sqlalchemy/sql/functions.py
index 7efb1e916..46f3e27dc 100644
--- a/lib/sqlalchemy/sql/functions.py
+++ b/lib/sqlalchemy/sql/functions.py
@@ -12,7 +12,7 @@ from . import sqltypes, schema
 from .base import Executable, ColumnCollection
 from .elements import ClauseList, Cast, Extract, _literal_as_binds, \
     literal_column, _type_from_args, ColumnElement, _clone,\
-    Over, BindParameter
+    Over, BindParameter, AggregateFilter
 from .selectable import FromClause, Select, Alias
 
 from . import operators
@@ -116,6 +116,28 @@ class FunctionElement(Executable, ColumnElement, FromClause):
         """
         return Over(self, partition_by=partition_by, order_by=order_by)
 
+    def filter(self, *criterion):
+        """Produce a FILTER clause against this function.
+
+        Used against aggregate functions,
+        for database backends that support aggregate "FILTER" clause.
+
+        The expression::
+
+            func.count(1).filter(True)
+
+        is shorthand for::
+
+            from sqlalchemy import aggregatefilter
+            aggregatefilter(func.count(1), True)
+
+        See :func:`~.expression.aggregatefilter` for a full description.
+
+        """
+        if not criterion:
+            return self
+        return AggregateFilter(self, *criterion)
+
     @property
     def _from_objects(self):
         return self.clauses._from_objects