The bad file contains records that were rejected, either by SQL*Loader or by the Oracle database. If you do not specify a bad file and there are rejected records, then SQL*Loader automatically creates one. It will have the same name as the data file, with a .bad extension. Some of the possible reasons for rejection are discussed in the next sections.
After a data file record is accepted for processing by SQL*Loader, it is sent to the database for insertion into a table as a row. If the database determines that the row is valid, then the row is inserted into the table. If the row is determined to be invalid, then the record is rejected and SQL*Loader puts it in the bad file. The row may be invalid, for example, because a key is not unique, because a required field is null, or because the field contains invalid data for the Oracle data type.
Case study 4, Loading Combined Physical Records (see "SQL*Loader Case Studies" for information on how to access case studies)