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8.2.2. 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 plays a part in the case sensitivity of database and table names. This means database and table names are not case sensitive in Windows, and case sensitive in most varieties of Unix. One notable exception is Mac OS X, which is Unix-based but uses a default file system 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.

MySQL Enterprise.  lower_case_table_names is just one of the system variables monitored by the MySQL Enterprise Monitor. For information about subscribing to this service, see http://www.mysql.com/products/enterprise/advisors.html.

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 and index names are not case sensitive on any platform, nor are column aliases.

Table aliases are case sensitive before MySQL 4.1.1. The following query would not work 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;

If you have trouble remembering the allowable lettercase for database and table names, 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 is 1 before MySQL 4.0.18 and 2 as of 4.0.18.

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. You should not set this variable to 0 if you are running MySQL on a system that has case-insensitive file names (such as Windows or Mac OS X). If you force this variable to 0 with --lower-case-table-names=0 on a case-insensitive file system and access MyISAM tablenames using different lettercases, this may lead to index corruption.
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 as of MySQL 4.0.2, and to table aliases as of 4.1.1.
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. This works only on file systems that are not case sensitive! InnoDB table names are stored in lowercase, as for lower_case_table_names=1. Setting lower_case_table_names to 2 can be done as of MySQL 4.0.18.

If you are using MySQL on only one platform, you do not normally have to change the lower_case_table_names variable from its default value. However, you may encounter difficulties if you want to transfer tables between platforms that differ in file system case sensitivity. For example, on Unix, you can have two different tables named my_table and MY_TABLE, but on Windows those names are considered identical. To avoid data transfer problems arising 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 do not 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 and you are trying to avoid these data transfer problems, you should set lower_case_table_names to 1 on all platforms to force names to be converted to lowercase.

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 stopping mysqld and restarting it with the new variable setting.


User Comments

Posted by Anders Eriksson on January 20 2009 9:31am[Delete] [Edit]

How to produce a SQL script that renames all tables in a schema to its lower case form:

select concat('rename table ', table_name, ' to ' , lower(table_name) , ';') from information_schema.tables where table_schema = 'your_schema_name';

/Anders Eriksson

Posted by Lone Wolf on December 6 2009 3:32am[Delete] [Edit]

Another way, via the command prompt:

cd /var/lib/mysql
for i in */*.frm; do DBASE="`dirname $i`"; TBL="`basename $i .frm`"; TBLl="`echo $TBL | tr A-Z a-z`"; if [[ "$TBL" != "$TBLl" ]] ; then echo "RENAME TABLE \`$DBASE\`.\`$TBL\` TO \`$DBASE\`.\`$TBLl\`" | mysql; fi; done

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