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11.4.1 The CHAR and VARCHAR Types
The CHAR
and VARCHAR
types
are similar, but differ in the way they are stored and
retrieved. They also differ in maximum length and in whether
trailing spaces are retained.
The CHAR
and VARCHAR
types
are declared with a length that indicates the maximum number of
characters you want to store. For example,
CHAR(30)
can hold up to 30 characters.
The length of a CHAR
column is fixed to the
length that you declare when you create the table. The length
can be any value from 0 to 255. When CHAR
values are stored, they are right-padded with spaces to the
specified length. When CHAR
values are
retrieved, trailing spaces are removed unless the
PAD_CHAR_TO_FULL_LENGTH
SQL
mode is enabled.
Values in VARCHAR
columns are variable-length
strings. The length can be specified as a value from 0 to
65,535. The effective maximum length of a
VARCHAR
is subject to the maximum row size
(65,535 bytes, which is shared among all columns) and the
character set used. See Section C.10.4, “Limits on Table Column Count and Row Size”.
In contrast to CHAR
,
VARCHAR
values are stored as a 1-byte or
2-byte length prefix plus data. The length prefix indicates the
number of bytes in the value. A column uses one length byte if
values require no more than 255 bytes, two length bytes if
values may require more than 255 bytes.
If strict SQL mode is not enabled and you assign a value to a
CHAR
or VARCHAR
column
that exceeds the column's maximum length, the value is truncated
to fit and a warning is generated. For truncation of nonspace
characters, you can cause an error to occur (rather than a
warning) and suppress insertion of the value by using strict SQL
mode. See Section 5.1.11, “Server SQL Modes”.
For VARCHAR
columns, trailing spaces in
excess of the column length are truncated prior to insertion and
a warning is generated, regardless of the SQL mode in use. For
CHAR
columns, truncation of excess trailing
spaces from inserted values is performed silently regardless of
the SQL mode.
VARCHAR
values are not padded when they are
stored. Trailing spaces are retained when values are stored and
retrieved, in conformance with standard SQL.
The following table illustrates the differences between
CHAR
and VARCHAR
by
showing the result of storing various string values into
CHAR(4)
and VARCHAR(4)
columns (assuming that the column uses a single-byte character
set such as latin1
).
Value | CHAR(4) |
Storage Required | VARCHAR(4) |
Storage Required |
---|---|---|---|---|
'' |
' ' |
4 bytes | '' |
1 byte |
'ab' |
'ab ' |
4 bytes | 'ab' |
3 bytes |
'abcd' |
'abcd' |
4 bytes | 'abcd' |
5 bytes |
'abcdefgh' |
'abcd' |
4 bytes | 'abcd' |
5 bytes |
The values shown as stored in the last row of the table apply only when not using strict mode; if MySQL is running in strict mode, values that exceed the column length are not stored, and an error results.
InnoDB
encodes fixed-length fields greater
than or equal to 768 bytes in length as variable-length fields,
which can be stored off-page. For example, a
CHAR(255)
column can exceed 768 bytes if the
maximum byte length of the character set is greater than 3, as
it is with utf8mb4
.
If a given value is stored into the CHAR(4)
and VARCHAR(4)
columns, the values retrieved
from the columns are not always the same because trailing spaces
are removed from CHAR
columns upon retrieval.
The following example illustrates this difference:
- Query OK, 0 rows affected (0.01 sec)
- Query OK, 1 row affected (0.00 sec)
- +---------------------+---------------------+
- +---------------------+---------------------+
- | (ab ) | (ab) |
- +---------------------+---------------------+
Values in CHAR
and VARCHAR
columns are sorted and compared according to the character set
collation assigned to the column.
Most MySQL collations have a pad attribute of PAD SPACE. The exceptions are Unicode collations based on UCA 9.0.0 and higher, which have a pad attribute of NO PAD. (see Section 10.10.1, “Unicode Character Sets”).
To determine the pad attribute for a collation, use the
INFORMATION_SCHEMA
COLLATIONS
table, which has a
PAD_ATTRIBUTE
column.
The pad attribute determines how trailing spaces are treated for
comparison of nonbinary strings (CHAR
,
VARCHAR
, and TEXT
values).
NO PAD collations treat spaces at the end of strings like any
other character. For PAD SPACE collations, trailing spaces are
insignificant in comparisons; strings are compared without
regard to any trailing spaces. “Comparison” in this
context does not include the LIKE
pattern-matching operator, for which trailing spaces are
significant. For example:
- Query OK, 0 rows affected (0.03 sec)
- Query OK, 1 row affected (0.00 sec)
- +------------------+--------------------+
- | myname = 'Monty' | myname = 'Monty ' |
- +------------------+--------------------+
- | 1 | 1 |
- +------------------+--------------------+
- +---------------------+-----------------------+
- +---------------------+-----------------------+
- | 1 | 0 |
- +---------------------+-----------------------+
This is true for all MySQL versions, and is not affected by the server SQL mode.
For more information about MySQL character sets and collations, see Chapter 10, Character Sets, Collations, Unicode. For additional information about storage requirements, see Section 11.8, “Data Type Storage Requirements”.
For those cases where trailing pad characters are stripped or
comparisons ignore them, if a column has an index that requires
unique values, inserting into the column values that differ only
in number of trailing pad characters will result in a
duplicate-key error. For example, if a table contains
'a'
, an attempt to store
'a '
causes a duplicate-key error.
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