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9.2.2. Identifier Case Sensitivity

In MySQL, databases correspond to directories within the data directory. Each table within a database corresponds to at least one file within the database directory (and possibly more, depending on the storage engine). Consequently, the case sensitivity of the underlying operating system determines the case sensitivity of database and table names. This means database and table names are case sensitive in most varieties of Unix, and not case sensitive in Windows. One notable exception is Mac OS X, which is Unix-based but uses a default filesystem type (HFS+) that is not case sensitive. However, Mac OS X also supports UFS volumes, which are case sensitive just as on any Unix. See Section 1.9.4, “MySQL Extensions to Standard SQL”. The lower_case_table_names system variable also affects how the server handles identifier case sensitivity, as described later in this section.

Note: Although database and table names are not case sensitive on some platforms, you should not refer to a given database or table using different cases within the same statement. The following statement would not work because it refers to a table both as my_table and as MY_TABLE:

mysql> SELECT * FROM my_table WHERE MY_TABLE.col=1;

Column, index, stored routine, and trigger names are not case sensitive on any platform, nor are column aliases.

By default, table aliases are case sensitive on Unix, but not so on Windows or Mac OS X. The following statement would not work on Unix, because it refers to the alias both as a and as A:

mysql> SELECT col_name FROM tbl_name AS a
    -> WHERE a.col_name = 1 OR A.col_name = 2;

However, this same statement is permitted on Windows. To avoid being caught out by such differences, it is best to adopt a consistent convention, such as always creating and referring to databases and tables using lowercase names. This convention is recommended for maximum portability and ease of use.

How table and database names are stored on disk and used in MySQL is affected by the lower_case_table_names system variable, which you can set when starting mysqld. lower_case_table_names can take the values shown in the following table. On Unix, the default value of lower_case_table_names is 0. On Windows the default value is 1. On Mac OS X, the default value is 2.

Value Meaning
0 Table and database names are stored on disk using the lettercase specified in the CREATE TABLE or CREATE DATABASE statement. Name comparisons are case sensitive. Note that if you force this variable to 0 with --lower-case-table-names=0 on a case-insensitive filesystem and access MyISAM tablenames using different lettercases, index corruption may result.
1 Table names are stored in lowercase on disk and name comparisons are not case sensitive. MySQL converts all table names to lowercase on storage and lookup. This behavior also applies to database names and table aliases.
2 Table and database names are stored on disk using the lettercase specified in the CREATE TABLE or CREATE DATABASE statement, but MySQL converts them to lowercase on lookup. Name comparisons are not case sensitive. Note: This works only on filesystems that are not case sensitive! InnoDB table names are stored in lowercase, as for lower_case_table_names=1.

If you are using MySQL on only one platform, you don't normally have to change the lower_case_table_names variable. However, you may encounter difficulties if you want to transfer tables between platforms that differ in filesystem case sensitivity. For example, on Unix, you can have two different tables named my_table and MY_TABLE, but on Windows these two names are considered identical. To avoid data transfer problems stemming from lettercase of database or table names, you have two options:

  • Use lower_case_table_names=1 on all systems. The main disadvantage with this is that when you use SHOW TABLES or SHOW DATABASES, you don't see the names in their original lettercase.

  • Use lower_case_table_names=0 on Unix and lower_case_table_names=2 on Windows. This preserves the lettercase of database and table names. The disadvantage of this is that you must ensure that your statements always refer to your database and table names with the correct lettercase on Windows. If you transfer your statements to Unix, where lettercase is significant, they do not work if the lettercase is incorrect.

    Exception: If you are using InnoDB tables, you should set lower_case_table_names to 1 on all platforms to force names to be converted to lowercase.

Note that if you plan to set the lower_case_table_names system variable to 1 on Unix, you must first convert your old database and table names to lowercase before restarting mysqld with the new variable setting.


 
 
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