Google BigQuery Connectors > Part II: Data Integration with Google BigQuery V2 Connector > Mappings and mapping tasks with Google BigQuery V2 Connector > Custom query source type
  

Custom query source type

You can use a custom query as a source object when you use a Google BigQuery V2 connection.
You might want to use a custom query as the source when a source object is large. You can use the custom query to reduce the number of fields that enter the data flow. You can also create a parameter for the source type when you design your mapping so that you can define the query in the Mapping Task wizard.
To use a custom query as a source, select Query as the source type when you configure the source transformation and then use valid and supported SQL to define the query.
You can use legacy SQL or standard SQL to define a custom query. To define a legacy SQL custom query, you must select the Use Legacy SQL For Custom Query option when you create a Google BigQuery V2 connection. You can unselect the Use Legacy SQL For Custom Query option to define a standard SQL custom query.
For more information about Google BigQuery Legacy SQL functions and operators, click the following URL:
https://cloud.google.com/bigquery/docs/reference/legacy-sql

Rules and Guidelines for Google BigQuery Custom Queries

Use the following rules and guidelines when you use a custom query as a source: