diff --git a/pydruid/utils/aggregators.py b/pydruid/utils/aggregators.py
index fd4c5a79..c61ab584 100644
--- a/pydruid/utils/aggregators.py
+++ b/pydruid/utils/aggregators.py
@@ -67,6 +67,8 @@ def count(raw_metric):
 def hyperunique(raw_metric):
     return {"type": "hyperUnique", "fieldName": raw_metric}
 
+def expression(raw_metric, aggregation):
+    return {"expression": "\""+raw_metric+"\"", "type":aggregation}
 
 def cardinality(raw_column, by_row=False):
     if type(raw_column) is not list: