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 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 1.8.4, “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, and
stored routine names are not case sensitive on any platform, nor are
column aliases. Trigger names are case sensitive, which differs from
standard SQL.
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 problems caused 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. 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, 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. 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. 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 these two 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. To do this for an individual table, use RENAME
TABLE:
RENAME TABLE T1 TO t1; To convert one or more entire databases, dump
them before setting lower_case_table_names, then drop the databases,
and reload them after setting lower_case_table_names:
Use mysqldump to dump each database:
mysqldump --databases db1 > db1.sql mysqldump --databases db2 >
db2.sql ... Do this for each database that must be recreated.
Use DROP DATABASE to drop each database.
Stop the server, set lower_case_table_names, and restart the server.
Reload the dump file for each database. Because lower_case_table_names
is set, each database and table name will be converted to lowercase as
it is recreated:
mysql < db1.sql mysql < db2.sql ... Object names may be considered
duplicates if their uppercase forms are equal according to a binary
collation. That is true for names of cursors, conditions, procedures,
functions, savepoints, stored routine parameters and stored program
local variables. It is not true for names of names of columns,
constraints, databases, statements prepared with PREPARE, tables,
triggers, users, and user-defined variables.
