9.2.3. Mapping of Identifiers to File Names

There is a correspondence between database and table identifiers and names in the file system. For the basic structure, MySQL represents each database as a directory in the data directory, and each table by one or more files in the appropriate database directory. For the table format files (.FRM), the data is always stored in this structure and location.

For the data and index files, the exact representation on disk is storage engine specific. These files may be stored in the same location as the FRM files, or the information may be stored in a separate file. InnoDB data is stored in the InnoDB data files. If you are using tablespaces with InnoDB, then the specific tablespace files you create are used instead.

Any character is legal in database or table identifiers except ASCII NUL (0x00). MySQL encodes any characters that are problematic in the corresponding file system objects when it creates database directories or table files:

Basic Latin letters (a..zA..Z), digits (0..9) and underscore (_) are encoded as is. Consequently, their case sensitivity directly depends on file system features.

All other national letters from alphabets that have uppercase/lowercase mapping are encoded as shown in the following table. Values in the Code Range column are UCS-2 values.

Code Range	Pattern	Number	Used	Unused	Blocks
00C0..017F	[@][0..4][g..z]	5*20= 100	97	3	Latin-1 Supplement + Latin Extended-A
0370..03FF	[@][5..9][g..z]	5*20= 100	88	12	Greek and Coptic
0400..052F	[@][g..z][0..6]	20*7= 140	137	3	Cyrillic + Cyrillic Supplement
0530..058F	[@][g..z][7..8]	20*2= 40	38	2	Armenian
2160..217F	[@][g..z][9]	20*1= 20	16	4	Number Forms
0180..02AF	[@][g..z][a..k]	20*11=220	203	17	Latin Extended-B + IPA Extensions
1E00..1EFF	[@][g..z][l..r]	20*7= 140	136	4	Latin Extended Additional
1F00..1FFF	[@][g..z][s..z]	20*8= 160	144	16	Greek Extended
.... ....	[@][a..f][g..z]	6*20= 120	0	120	RESERVED
24B6..24E9	[@][@][a..z]	26	26	0	Enclosed Alphanumerics
FF21..FF5A	[@][a..z][@]	26	26	0	Halfwidth and Fullwidth forms

One of the bytes in the sequence encodes lettercase. For example: LATIN CAPITAL LETTER A WITH GRAVE is encoded as @0G, whereas LATIN SMALL LETTER A WITH GRAVE is encoded as @0g. Here the third byte (G or g) indicates lettercase. (On a case-insensitive file system, both letters will be treated as the same.)

For some blocks, such as Cyrillic, the second byte determines lettercase. For other blocks, such as Latin1 Supplement, the third byte determines lettercase. If two bytes in the sequence are letters (as in Greek Extended), the leftmost letter character stands for lettercase. All other letter bytes must be in lowercase.

All nonletter characters except underscore (_), as well as letters from alphabets that do not have uppercase/lowercase mapping (such as Hebrew) are encoded using hexadecimal representation using lowercase letters for hex digits a..f:
```
0x003F -> @003f0xFFFF -> @ffff
```
The hexadecimal values correspond to character values in the ucs2 double-byte character set.

On Windows, some names such as nul, prn, and aux are encoded by appending @@@ to the name when the server creates the corresponding file or directory. This occurs on all platforms for portability of the corresponding database object between platforms.

If you have databases or tables from a version of MySQL older than 5.1.6 that contain special characters and for which the underlying directory names or file names have not been updated to use the new encoding, the server displays their names with a prefix of #mysql50# in the output from INFORMATION_SCHEMA tables or SHOW statements. For example, if you have a table named a@b and its name encoding has not been updated, SHOW TABLES displays it like this:

mysql> SHOW TABLES;+----------------+| Tables_in_test |+----------------+| #mysql50#a@b   |+----------------+

To refer to such a name for which the encoding has not been updated, you must supply the #mysql50# prefix:

mysql> SHOW COLUMNS FROM `a@b`;ERROR 1146 (42S02): Table 'test.a@b' doesn't existmysql> SHOW COLUMNS FROM `#mysql50#a@b`;+-------+---------+------+-----+---------+-------+| Field | Type    | Null | Key | Default | Extra |+-------+---------+------+-----+---------+-------+| i     | int(11) | YES  |     | NULL    |       |+-------+---------+------+-----+---------+-------+

To update old names to eliminate the need to use the special prefix to refer to them, re-encode them with mysqlcheck. The following command updates all names to the new encoding:

shell> mysqlcheck --check-upgrade --fix-db-names
        --fix-table-names --all-databases

To check only specific databases or tables, omit --all-databases and provide the appropriate database or table arguments. For information about mysqlcheck invocation syntax, see Section 4.5.3, "mysqlcheck — A Table Maintenance Program".

Note

The #mysql50# prefix is intended only to be used internally by the server. You should not create databases or tables with names that use this prefix.

Also, mysqlcheck cannot fix names that contain literal instances of the @ character that is used for encoding special characters. If you have databases or tables that contain this character, use mysqldump to dump them before upgrading to MySQL 5.1.6 or later, and then reload the dump file after upgrading.