MySQL 5.7 Release Notes
String columns in INFORMATION_SCHEMA tables
have a collation of utf8_general_ci, which is
case-insensitive. However, for values that correspond to objects
that are represented in the file system, such as databases and
tables, searches in INFORMATION_SCHEMA string
columns can be case-sensitive or case-insensitive, depending on
the characteristics of the underlying file system and the value
of the lower_case_table_names
system variable. For example, searches may be case-sensitive if
the file system is case-sensitive. This section describes this
behavior and how to modify it if necessary; see also Bug #34921.
Suppose that a query searches the
SCHEMATA.SCHEMA_NAME column for the
test database. On Linux, file systems are
case-sensitive, so comparisons of
SCHEMATA.SCHEMA_NAME with
'test' match, but comparisons with
'TEST' do not:
mysql> SELECT SCHEMA_NAME FROM INFORMATION_SCHEMA.SCHEMATA
WHERE SCHEMA_NAME = 'test';
+-------------+
| SCHEMA_NAME |
+-------------+
| test |
+-------------+
mysql> SELECT SCHEMA_NAME FROM INFORMATION_SCHEMA.SCHEMATA
WHERE SCHEMA_NAME = 'TEST';
Empty set (0.00 sec)
These results occur with the
lower_case_table_names system
variable set to 0. Changing the value of
lower_case_table_names to 1 or
2 causes the second query to return the same (nonempty) result
as the first query.
On Windows or macOS, file systems are not case-sensitive, so
comparisons match both 'test' and
'TEST':
mysql> SELECT SCHEMA_NAME FROM INFORMATION_SCHEMA.SCHEMATA
WHERE SCHEMA_NAME = 'test';
+-------------+
| SCHEMA_NAME |
+-------------+
| test |
+-------------+
mysql> SELECT SCHEMA_NAME FROM INFORMATION_SCHEMA.SCHEMATA
WHERE SCHEMA_NAME = 'TEST';
+-------------+
| SCHEMA_NAME |
+-------------+
| TEST |
+-------------+
The value of
lower_case_table_names makes no
difference in this context.
The preceding behavior occurs because the
utf8_general_ci collation is not used for
INFORMATION_SCHEMA queries when searching for
values that correspond to objects represented in the file
system. It is a result of file system-scanning optimizations
implemented for INFORMATION_SCHEMA searches.
For information about these optimizations, see
Section 8.2.3, “Optimizing INFORMATION_SCHEMA Queries”.
If the result of a string operation on an
INFORMATION_SCHEMA column differs from
expectations, a workaround is to use an explicit
COLLATE clause to force a suitable collation
(see Section 10.8.1, “Using COLLATE in SQL Statements”). For example, to perform
a case-insensitive search, use COLLATE with
the INFORMATION_SCHEMA column name:
mysql> SELECT SCHEMA_NAME FROM INFORMATION_SCHEMA.SCHEMATA
WHERE SCHEMA_NAME COLLATE utf8_general_ci = 'test';
+-------------+
| SCHEMA_NAME |
+-------------+
| test |
+-------------+
mysql> SELECT SCHEMA_NAME FROM INFORMATION_SCHEMA.SCHEMATA
WHERE SCHEMA_NAME COLLATE utf8_general_ci = 'TEST';
+-------------+
| SCHEMA_NAME |
+-------------+
| test |
+-------------+
In the preceding queries, it is important to apply the
COLLATE clause to the
INFORMATION_SCHEMA column name. Applying
COLLATE to the comparison value has no
effect.
You can also use the UPPER() or
LOWER() function:
WHERE UPPER(SCHEMA_NAME) = 'TEST'
WHERE LOWER(SCHEMA_NAME) = 'test'
Although a case-insensitive comparison can be performed even on
platforms with case-sensitive file systems, as just shown, it is
not necessarily always the right thing to do. On such platforms,
it is possible to have multiple objects with names that differ
only in lettercase. For example, tables named
city, CITY, and
City can all exist simultaneously. Consider
whether a search should match all such names or just one and
write queries accordingly. The first of the following
comparisons (with utf8_bin) is
case-sensitive; the others are not:
WHERE TABLE_NAME COLLATE utf8_bin = 'City'
WHERE TABLE_NAME COLLATE utf8_general_ci = 'city'
WHERE UPPER(TABLE_NAME) = 'CITY'
WHERE LOWER(TABLE_NAME) = 'city'
Searches in INFORMATION_SCHEMA string columns
for values that refer to INFORMATION_SCHEMA
itself do use the utf8_general_ci collation
because INFORMATION_SCHEMA is a
“virtual” database not represented in the file
system. For example, comparisons with
SCHEMATA.SCHEMA_NAME match
'information_schema' or
'INFORMATION_SCHEMA' regardless of platform:
mysql> SELECT SCHEMA_NAME FROM INFORMATION_SCHEMA.SCHEMATA
WHERE SCHEMA_NAME = 'information_schema';
+--------------------+
| SCHEMA_NAME |
+--------------------+
| information_schema |
+--------------------+
mysql> SELECT SCHEMA_NAME FROM INFORMATION_SCHEMA.SCHEMATA
WHERE SCHEMA_NAME = 'INFORMATION_SCHEMA';
+--------------------+
| SCHEMA_NAME |
+--------------------+
| information_schema |
+--------------------+