User-defined aggregate functions (UDAFs) that support SQL expressions are in preview

## Feature [User-defined aggregate functions (UDAFs)](https://cloud.google.com/bigquery/docs/user-defined-aggregates) that support SQL expressions are in [preview](https://cloud.google.com/products#product-launch-stages). You can create a UDAF with the [CREATE AGGREGATE FUNCTION](https://cloud.google.com/bigquery/docs/reference/standard-sql/data-definition-language#sql-create-udaf-function) statement.