Javadoc
[Optional] Specifies the action that occurs if the destination table already exists. The
following values are supported: WRITE_TRUNCATE: If the table already exists, BigQuery
overwrites the table data and uses the schema from the query result. WRITE_APPEND: If the table
already exists, BigQuery appends the data to the table. WRITE_EMPTY: If the table already
exists and contains data, a 'duplicate' error is returned in the job result. The default value
is WRITE_EMPTY. Each action is atomic and only occurs if BigQuery is able to complete the job
successfully. Creation, truncation and append actions occur as one atomic update upon job
completion.