Update index.md

Author: joeynmq

src/connections/reverse-etl/index.md

@@ -321,3 +321,6 @@ To receive Reverse ETL sync notifications:
 3. Enable the toggle for **Reverse ETL Sync Failed**.
 
 In case of consecutive failures, Segment sends notifications for every sync failure. Segment doesn't send notifications for partial failures.
+
+#### Can I have multiple queries in the Query Builder?
+No. In Reverse ETL, we execute queries in a [common table expression](https://cloud.google.com/bigquery/docs/reference/standard-sql/query-syntax#with_clause), which can only bind the results from one single subquery. If there are multiple semicolons `;` in the query, they'll also be treated as several subqueries (even if the second part is only an inline comment), and cause the syntax error.