[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. 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_APPEND. 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.