The default character set for all data files, if the CHARACTERSET parameter is not specified, is the session character set defined by the NLS_LANG parameter. The character set used in input data files can be specified with the CHARACTERSET parameter.
SQL*Loader can automatically convert data from the data file character set to the database character set or the database national character set, when they differ.
When data character set conversion is required, the target character set should be a superset of the source data file character set. Otherwise, characters that have no equivalent in the target character set are converted to replacement characters, often a default character such as a question mark (?). This causes loss of data.
The sizes of the database character types CHAR and VARCHAR2 can be specified in bytes (byte-length semantics) or in characters (character-length semantics). If they are specified in bytes, and data character set conversion is required, then the converted values may take more bytes than the source values if the target character set uses more bytes than the source character set for any character that is converted. This will result in the following error message being reported if the larger target value exceeds the size of the database column:
ORA-01401: inserted value too large for column
You can avoid this problem by specifying the database column size in characters and also by using character sizes in the control file to describe the data. Another way to avoid this problem is to ensure that the maximum column size is large enough, in bytes, to hold the converted value.