One of the most common NLS-related problems reported to Oracle Worldwide
Customer Support is the loss or changing of characters after an export and
import. This is almost always due to NLS_LANG being set to the incorrect
character set during export.

Role of NLS_LANG During Export/Import:

This is explained in detail in Note 15095.1 but, in brief, export and import
are client tools and will work under the character set specified by NLS_LANG.
If, for instance, the database is created with a character set of WE8DEC and
NLS_LANG is set to AMERICAN_AMERICA.WE8PC850 then the ascii values of the stored
characters in the database will be translated to the values for the same
characters in the WE8PC850 character set. The character set defined by NLS_LANG
during the export is stored in the export dump file and is used to ensure that
the data is translated correctly to the character set defined by NLS_LANG for
the import.

Potential Problems:

If NLS_LANG is not set, for instance, export will be done under US7ASCII, the
default character set. If the database was built with character set WE8DEC
the characters stored in the database will be converted to US7ASCII and any
8-bit characters, having no equivalent in US7ASCII, will be stripped out.

The same problem will be seen if the character set defined by NLS_LANG is
not a superset of the one being translated from (ie: the database character
set on export or the export file character set on import).

Identifying the Export Character Set:

When investigating problems like these it is useful to check the character
set used for the export. As said above, this is held in the export dump file.
It can be seen by doing a hex dump of the export file as follows (in Unix):

cat expdat.dmp | od -x | head

This will produce output similar to:

0000000 0300 0145 5850 4f52 543a 5630 372e 3033
0000020 2e30 330a 4454 534f 0a52 5441 424c 4553
0000040 0a31 3032 340a 300a 0020 2020 2020 2020
0000060 2020 2020 2020 2020 2020 2020 2020 2020
*
0000140 2020 2020 2020 2020 4d6f 6e20 4e6f 7620
0000160 3130 2031 343a 3031 3a33 3620 3139 3937
0000200 0a54 4142 4c45 2022 454d 5022 0a43 5245
0000220 4154 4520 5441 424c 4520 2245 4d50 2220

The second and the third byte in the file define the character set used for
the export.

In the example above, the second byte is 0x00 and the third byte is 0x01,
yielding 0x0001 as the character set ID. This shows that NLS_LANG was set
to US7ASCII during the export. The new Oracle8 functions NLS_CHARSET_NAME
and NLS_CHARSET_ID can be used to map character set IDs to character set names.
The mapping is also given in Note 13971.1.

Note that the 16-bit value is stored in the EXP platform endian.

Most unix platforms are big-endian ( Sparc, PowerPc, PARisc, RS/6000, SGI R4000 systems),
 i.e. the most significant byte is showed first.
(like above example -> if the file begins with 03xx -> big endian)

On little-endian platforms, (platforms running on Intel/AMD x86 and Alpha mainly)
the output will be slightly different as below:

00000000 0003 4501 5058 524f 3a54 2e37 3330
etc.

Here the most significant byte is showed *last* (!)

(if the file begins with xx03 -> little endian)

The values for the most commonly used character sets are below:

Name            ID
----------------------
US7ASCII        0x0001
WE8DEC    0x0002
WE8ISO8859P1    0x001f
EE8ISO8859P2    0x0020
SE8ISO8859P3    0x0021
NE8ISO8850P4    0x0022
CL8ISO8859P5    0x0023
AR8ISO8859P6    0x0024
EL8ISO8859P7    0x0025
IW8ISO8859P8    0x0026
WE8ISO8859P9    0x0027
WE8ISO8859P15   0x002e
TH8TISASCII     0x0029
US8PC437        0x0004
WE8ROMAN8       0x0005
WE8PC850        0x000a
EE8PC852        0x0096
RU8PC855        0X009B         
TR8PC857        0x009C         
WE8PC858        0x001c
WE8PC860        0x00A0
IS8PC861        0x00A1         
N8PC865         0x00BE         
RU8PC866        0x0098
EE8MSWIN1250    0x00aa
CL8MSWIN1251    0x00ab
WE8MSWIN1252    0x00b2
EL8MSWIN1253    0x00ae
TR8MSWIN1254    0x00b1
IW8MSWIN1255    0x00af         
AR8MSWIN1256    0x0230
BLT8MSWIN1257   0x00b3
ZHT16MSWIN950   0x0363
ZHS16GBK        0x0354
ZHT16HKSCS      0x0364
JA16EUC  0x033e
JA16SJIS        0x0340
ZHT16BIG5       0x0361
AL24UTFFSS      0x0366
UTF8            0x0367
AL32UTF8        0x0369

        select nls_charset_id(value) nls_charset_id,  value
        from  v$nls_valid_values
        where parameter = 'CHARACTERSET'
        order by nls_charset_id(value);

Gives the nls_charset_id in DECIMAL, so you need to convert it to HEX first.

Alternative you can open the characterset definition using Locale Builder (9i
and up), this will also show the characterset ID in DECIMAL in the first screen
(note that there is also an ISO ID that is NOT used here in the exp file)
Note 223706.1  Using Locale Builder to view the definition of character sets

Warning:  User modifications of export dump files are not supported
========  by Oracle.  The character set information is also held in other
          places in the export dump file and modifying only the two bytes
          may lead to problems with imported data.

          NOT WORKING any more with Oracle 9i R2 (9.2) and up,
          due to changes in the import /export tools
          but you can use a 8i exp against a 9i db for example, more
          info is in Note 132904.1 Compatibility Matrix for Export & Import
                                     Between Different Oracle Versions

In some cases it can be useful to modify the character set information
held in the dump file. This should not be taken lightly since the character
set information is also held in other places.

We STRONGLY advice you to log a NLS tar FIRST to get confirmation that this is
a solution for your problem before starting to change this header.

If, after careful consideration of other options and verification by support,
you do decide to edit the character set simply use a binary file editor to do so.

You could for example use a freeware Hex Editor like
* XVI32 http://www.chmaas.handshake.de/delphi/freeware/xvi32/xvi32.htm
* HxD  http://mh-nexus.de/en/hxd/

In case of multiple dump files from a single export, you need to modify each
and every export dump file. Otherwise, you will error out with -
IMP-00008: unrecognized statement in the export file:
when opening the second file.

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