11.4. String Types

The CHAR and VARCHAR types are similar, but differ in the way they are stored and retrieved. As of MySQL 5.0.3, 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.

Values in VARCHAR columns are variable-length strings. The length can be specified as a value from 0 to 255 before MySQL 5.0.3, and 0 to 65,535 in 5.0.3 and later versions. (The maximum effective length of a VARCHAR in MySQL 5.0.3 and later is determined by the maximum row size and the character set used. The maximum length overall is 65,532 bytes.)

In contrast to CHAR, VARCHAR values are stored using only as many characters as are needed, plus one byte to record the length (two bytes for columns that are declared with a length longer than 255).

VARCHAR values are not padded when they are stored. Handling of trailing spaces is version-dependent. As of MySQL 5.0.3, trailing spaces are retained when values are stored and retrieved, in conformance with standard SQL. Before MySQL 5.0.3, trailing spaces are removed from values when they are stored into a VARCHAR column; this means that the spaces also are absent from retrieved values.

If you assign a value to a CHAR or VARCHAR column that exceeds the column's maximum length, the value is truncated to fit. If the truncated characters are not spaces, a warning is generated. For truncation of non-space 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 5.2.5, “The Server SQL Mode”.

Before MySQL 5.0.3, if you need a data type for which trailing spaces are not removed, consider using a BLOB or TEXT type. Also, if you want to store binary values such as results from an encryption or compression function that might contain arbitrary byte values, use a BLOB column rather than a CHAR or VARCHAR column, to avoid potential problems with trailing space removal that would change data values.

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:

Note that 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.

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:

mysql> CREATE TABLE vc (v VARCHAR(4), c CHAR(4));
Query OK, 0 rows affected (0.01 sec)

mysql> INSERT INTO vc VALUES ('ab  ', 'ab  ');
Query OK, 1 row affected (0.00 sec)

mysql> SELECT CONCAT('(', v, ')'), CONCAT('(', c, ')') FROM vc;
+---------------------+---------------------+
| CONCAT('(', v, ')') | CONCAT('(', c, ')') |
+---------------------+---------------------+
| (ab  )              | (ab)                |
+---------------------+---------------------+
1 row in set (0.06 sec)

Values in CHAR and VARCHAR columns are sorted and compared according to the character set collation assigned to the column.

Note that all MySQL collations are of type PADSPACE. This means that all CHAR and VARCHAR values in MySQL are compared without regard to any trailing spaces. For example:

mysql> CREATE TABLE names (myname CHAR(10), yourname VARCHAR(10));
Query OK, 0 rows affected (0.09 sec)

mysql> INSERT INTO names VALUES ('Monty ', 'Monty ');
Query OK, 1 row affected (0.00 sec)

mysql> SELECT myname = 'Monty  ', yourname = 'Monty  ' FROM names;
+--------------------+----------------------+
| myname = 'Monty  ' | yourname = 'Monty  ' |
+--------------------+----------------------+
|                  1 |                    1 |
+--------------------+----------------------+
1 row in set (0.00 sec)

Note that this is true for all MySQL versions, and it makes no difference whether your version trims trailing spaces from VARCHAR values before storing them. Nor does the server SQL mode make any difference in this regard.

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.

The BINARY and VARBINARY types are similar to CHAR and VARCHAR, except that they contain binary strings rather than non-binary strings. That is, they contain byte strings rather than character strings. This means that they have no character set, and sorting and comparison are based on the numeric values of the bytes in the values.

The allowable maximum length is the same for BINARY and VARBINARY as it is for CHAR and VARCHAR, except that the length for BINARY and VARBINARY is a length in bytes rather than in characters.

The BINARY and VARBINARY data types are distinct from the CHAR BINARY and VARCHAR BINARY data types. For the latter types, the BINARY attribute does not cause the column to be treated as a binary string column. Instead, it causes the binary collation for the column character set to be used, and the column itself contains non-binary character strings rather than binary byte strings. For example, CHAR(5) BINARY is treated as CHAR(5) CHARACTER SET latin1 COLLATE latin1_bin, assuming that the default character set is latin1. This differs from BINARY(5), which stores 5-bytes binary strings that have no character set or collation.

When BINARY values are stored, they are right-padded with the pad value to the specified length. The pad value and how it is handled is version specific:

For VARBINARY, there is no padding on insert and no bytes are stripped on select. All bytes are significant in comparisons, including ORDER BY and DISTINCT operations. 0x00 bytes and spaces are different in comparisons, with 0x00 < space. (Exceptions: Before MySQL 5.0.3, trailing spaces are removed when values are stored. Before MySQL 5.0.15, trailing 0x00 bytes are removed for ORDER BY operations.)

Note: The InnoDB storage engine continues to preserve trailing spaces in BINARY and VARBINARY column values through MySQL 5.0.18. Beginning with MySQL 5.0.19, InnoDB uses trailing space characters in making comparisons as do other MySQL storage engines.

For those cases where trailing pad bytes 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 bytes will result in a duplicate-key error. For example, if a table contains 'a', an attempt to store 'a\0' causes a duplicate-key error.

You should consider the preceding padding and stripping characteristics carefully if you plan to use the BINARY data type for storing binary data and you require that the value retrieved be exactly the same as the value stored. The following example illustrates how 0x00-padding of BINARY values affects column value comparisons:

mysql> CREATE TABLE t (c BINARY(3));
Query OK, 0 rows affected (0.01 sec)

mysql> INSERT INTO t SET c = 'a';
Query OK, 1 row affected (0.01 sec)

mysql> SELECT HEX(c), c = 'a', c = 'a\0\0' from t;
+--------+---------+-------------+
| HEX(c) | c = 'a' | c = 'a\0\0' |
+--------+---------+-------------+
| 610000 |       0 |           1 |
+--------+---------+-------------+
1 row in set (0.09 sec)

If the value retrieved must be the same as the value specified for storage with no padding, it might be preferable to use VARBINARY or one of the BLOB data types instead.

A BLOB is a binary large object that can hold a variable amount of data. The four BLOB types are TINYBLOB, BLOB, MEDIUMBLOB, and LONGBLOB. These differ only in the maximum length of the values they can hold. The four TEXT types are TINYTEXT, TEXT, MEDIUMTEXT, and LONGTEXT. These correspond to the four BLOB types and have the same maximum lengths and storage requirements. See Section 11.5, “Data Type Storage Requirements”. No lettercase conversion for TEXT or BLOB columns takes place during storage or retrieval.

BLOB columns are treated as binary strings (byte strings). TEXT columns are treated as non-binary strings (character strings). BLOB columns have no character set, and sorting and comparison are based on the numeric values of the bytes in column values. TEXT columns have a character set, and values are sorted and compared based on the collation of the character set.

If a TEXT column is indexed, index entry comparisons are space-padded at the end. This means that, if the index requires unique values, duplicate-key errors will occur for values that differ only in the number of trailing spaces. For example, if a table contains 'a', an attempt to store 'a ' causes a duplicate-key error. This is not true for BLOB columns.

When not running in strict mode, if you assign a value to a BLOB or TEXT column that exceeds the data type's maximum length, the value is truncated to fit. If the truncated characters are not spaces, a warning is generated. You can cause an error to occur and the value to be rejected rather than to be truncated with a warning by using strict SQL mode. See Section 5.2.5, “The Server SQL Mode”.

In most respects, you can regard a BLOB column as a VARBINARY column that can be as large as you like. Similarly, you can regard a TEXT column as a VARCHAR column. BLOB and TEXT differ from VARBINARY and VARCHAR in the following ways:

LONG and LONG VARCHAR map to the MEDIUMTEXT data type. This is a compatibility feature. If you use the BINARY attribute with a TEXT data type, the column is assigned the binary collation of the column character set.

MySQL Connector/ODBC defines BLOB values as LONGVARBINARY and TEXT values as LONGVARCHAR.

Because BLOB and TEXT values can be extremely long, you might encounter some constraints in using them:

Each BLOB or TEXT value is represented internally by a separately allocated object. This is in contrast to all other data types, for which storage is allocated once per column when the table is opened.

In some cases, it may be desirable to store binary data such as media files in BLOB or TEXT columns. You may find MySQL's string handling functions useful for working with such data. See Section 12.3, “String Functions”. For security and other reasons, it is usually preferable to do so using application code rather than allowing application users the FILE privilege. You can discuss specifics for various languages and platforms in the MySQL Forums (http://forums.mysql.com/).

CREATE TABLE sizes (
    name ENUM('small', 'medium', 'large')
);

CREATE TABLE sizes (
    c1 ENUM('small', CONCAT('med','ium'), 'large')
);

SET @mysize = 'medium';

CREATE TABLE sizes (
    name ENUM('small', @mysize, 'large')
);

mysql> SELECT enum_col+0 FROM tbl_name;

numbers ENUM('0','1','2')

''
'one'
'two'
'one,two'

mysql> SELECT set_col+0 FROM tbl_name;

mysql> CREATE TABLE myset (col SET('a', 'b', 'c', 'd'));

mysql> INSERT INTO myset (col) VALUES 
-> ('a,d'), ('d,a'), ('a,d,a'), ('a,d,d'), ('d,a,d');
Query OK, 5 rows affected (0.01 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql> SELECT col FROM myset;
+------+
| col  |
+------+
| a,d  |
| a,d  |
| a,d  |
| a,d  |
| a,d  |
+------+
5 rows in set (0.04 sec)

mysql> INSERT INTO myset (col) VALUES ('a,d,d,s');
Query OK, 1 row affected, 1 warning (0.03 sec)

mysql> SHOW WARNINGS;
+---------+------+------------------------------------------+
| Level   | Code | Message                                  |
+---------+------+------------------------------------------+
| Warning | 1265 | Data truncated for column 'col' at row 1 |
+---------+------+------------------------------------------+
1 row in set (0.04 sec)

mysql> SELECT col FROM myset;
+------+
| col  |
+------+
| a,d  |
| a,d  |
| a,d  |
| a,d  |
| a,d  |
| a,d  |
+------+
6 rows in set (0.01 sec)

mysql> SELECT * FROM tbl_name WHERE FIND_IN_SET('value',set_col)>0;
mysql> SELECT * FROM tbl_name WHERE set_col LIKE '%value%';

mysql> SELECT * FROM tbl_name WHERE set_col & 1;
mysql> SELECT * FROM tbl_name WHERE set_col = 'val1,val2';