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MySQL 5.5 Reference Manual
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10.13.2 Choosing a Collation ID

Each collation must have a unique ID. To add a collation, you must choose an ID value that is not currently used. The range of IDs from 1024 to 2047 is reserved for user-defined collations. Before MySQL 5.5, an ID must be chosen from the range 1 to 254. As of MySQL 5.5, the 254 limit is removed for MyISAM tables with the introduction of support for two-byte collation IDs. Two-byte collation ID support for InnoDB tables is not added until MySQL 5.6.

The collation ID that you choose will appear in these contexts:

To determine the largest currently used ID, issue the following statement:

mysql> SELECT MAX(ID) FROM INFORMATION_SCHEMA.COLLATIONS;
+---------+
| MAX(ID) |
+---------+
|     210 |
+---------+

To display a list of all currently used IDs, issue this statement:

mysql> SELECT ID FROM INFORMATION_SCHEMA.COLLATIONS ORDER BY ID;
+-----+
| ID  |
+-----+
|   1 |
|   2 |
| ... |
|  52 |
|  53 |
|  57 |
|  58 |
| ... |
|  98 |
|  99 |
| 128 |
| 129 |
| ... |
| 210 |
+-----+
Warning

Before upgrading, you should save the configuration files that you change. If you upgrade in place, the process will replace the your modified files.