CHAR

Fixed-length character string type. CHAR columns store strings of a specified length (0 to 255), padding with spaces if necessary.

This page covers the CHAR data type. See CHAR Function for the function.

Syntax

[NATIONAL] CHAR[(M)] [CHARACTER SET charset_name] [COLLATE collation_name]

Description

A fixed-length string that is always right-padded with spaces to the specified length when stored. M represents the column length in characters. The range of M is 0 to 255. If M is omitted, the length is 1.

Trailing spaces are removed when CHAR values are retrieved unless the PAD_CHAR_TO_FULL_LENGTH SQL mode is enabled.

If a unique index consists of a column where trailing pad characters are stripped or ignored, inserts into that column where values differ only by the number of trailing pad characters will result in a duplicate-key error.

CHAR(0) columns can contain 2 values: an empty string or NULL. Such columns cannot be part of an index. The CONNECT storage engine does not support CHAR(0).

Use Cases for Zero Length

A CHAR(0) or VARCHAR(0) column occupies minimal space and is restricted to two possible values: an empty string ('') or NULL. You can use these columns for the following purposes:

• Legacy Compatibility: Include these columns to maintain compatibility with older applications that require a specific table schema, even if the data is no longer collected.

• Two-State Flags: A CHAR(0) NULL column can function as a boolean indicator. It uses only one bit of storage to distinguish between a "set" state (the empty string) and an "unset" state (NULL).

• Row Marking: You can use a CHAR(0) column to mark a specific row in a table. For example, if you require only one "active" row, set that row to an empty string while keeping all other rows NULL.

The following error occurs if you attempt to insert any character data into a 0-length column: ERROR 1406 (22001): Data too long for column.

Examples

Trailing spaces:

CREATE TABLE strtest (c CHAR(10));
INSERT INTO strtest VALUES('Maria   ');

SELECT c='Maria',c='Maria   ' FROM strtest;
+-----------+--------------+
| c='Maria' | c='Maria   ' |
+-----------+--------------+
|         1 |            1 |
+-----------+--------------+

SELECT c LIKE 'Maria',c LIKE 'Maria   ' FROM strtest;
+----------------+-------------------+
| c LIKE 'Maria' | c LIKE 'Maria   ' |
+----------------+-------------------+
|              1 |                 0 |
+----------------+-------------------+

Example of CHAR:

CREATE TABLE char_example (
   description VARCHAR(20),
   example CHAR(255)
) DEFAULT CHARSET=latin1; -- One byte per char makes the examples clearer

INSERT INTO char_example VALUES
   ('Normal foo', 'foo'),
   ('Trailing spaces foo', 'foo      '),
   ('NULLed', NULL),
   ('Empty', ''),
   ('Maximum', RPAD('', 255, 'x'));

SELECT description, LENGTH(example) AS length FROM char_example;

+---------------------+--------+
| description         | length |
+---------------------+--------+
| Normal foo          |      3 |
| Trailing spaces foo |      3 |
| NULLed              |   NULL |
| Empty               |      0 |
| Maximum             |    255 |
+---------------------+--------+

Data Too Long

When SQL_MODE is strict (the default), a value is considered "too long" when its length exceeds the size of the data type, and an error is generated.

Example of data too long behavior for CHAR:

TRUNCATE char_example;

INSERT INTO char_example VALUES
   ('Overflow', RPAD('', 256, 'x'));

ERROR 1406 (22001): Data too long for column 'example' at row 1

NO PAD Collations

NO PAD collations regard trailing spaces as normal characters. You can get a list of all NO PAD collations by querying the Information Schema Collations table, for example:

SELECT collation_name FROM information_schema.collations 
  WHERE collation_name LIKE "%nopad%";  
+------------------------------+
| collation_name               |
+------------------------------+
| big5_chinese_nopad_ci        |
| big5_nopad_bin               |
...

See Also

• CHAR Function

• VARCHAR

• BINARY

• Data Type Storage Requirements

_{This page is licensed: GPLv2, originally from fill_help_tables.sql}