The primary objective of using this payload is . For a UNION operation to work, the injected query must have the exact same number of columns as the original query.
The payload you provided, ' UNION ALL SELECT NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL-- , is a common pattern used in to determine the number of columns returned by an original database query. What this Payload Does The primary objective of using this payload is
: Attempts to select 8 columns of "null" data. ' UNION ALL SELECT NULL