[core-spec]: add FILTER (WHERE ...) aggregate modifier

core-spec/expression_language.md

@@ -217,7 +217,7 @@ APPROX_PERCENTILE(response_time, 0.95)
 ### Conditional Aggregations (REQUIRED)
 
 SUM / COUNT aggregation functions support `DISTINCT.`   
-All aggregations should support filtered aggregation:
+All aggregations must support filtered aggregation, expressed either with a `CASE` argument or with a postfix `FILTER (WHERE ...)` modifier:
 
 ```sql
 -- DISTINCT modifier
@@ -227,6 +227,32 @@ COUNT(DISTINCT customer_id)
 -- Filtered aggregation via CASE
 SUM(CASE WHEN status = 'completed' THEN amount ELSE 0 END)
 COUNT(CASE WHEN status = 'completed' THEN 1 END)
+
+-- Filtered aggregation via FILTER (WHERE ...)
+SUM(amount) FILTER (WHERE status = 'completed')
+COUNT(*) FILTER (WHERE status = 'completed')
+```
+
+#### `FILTER (WHERE ...)` (REQUIRED)
+
+Every aggregate function must support a postfix `FILTER (WHERE <predicate>)` modifier:
+
+```
+<aggregate_function>(<args>) FILTER (WHERE <predicate>)
+```
+
+The clause has the semantics of the SQL:2003 `<filter clause>` (optional feature T612): the aggregate considers only the rows for which `<predicate>` succeeds. `FILTER` is applied to the aggregate's input, not as a query `WHERE`, so it never removes output groups.
+
+`FILTER (WHERE ...)` is a modifier on an aggregate expression. It is not the standalone `WHERE` clause listed under [Not Supported in Expressions](#not-supported-in-expressions). The `<predicate>` must reference only fields of the same dataset as the aggregate's arguments.
+
+Engines without native `FILTER (WHERE ...)` support MAY lower it to the equivalent `CASE` form:
+
+```sql
+-- value aggregate: filter the argument
+SUM(amount) FILTER (WHERE status = 'completed')  --> SUM(CASE WHEN status = 'completed' THEN amount END)
+
+-- COUNT(*): filter a constant
+COUNT(*) FILTER (WHERE status = 'completed')      --> COUNT(CASE WHEN status = 'completed' THEN 1 END)
 ```
 
 ### Decomposability Reference