[NATIONAL] VARCHAR(M) [CHARACTER SET charset_name] [COLLATE collation_name]A variable-length string. M represents the maximum column length in characters. The range of M is 0 to 65,532. The effective maximum length of a VARCHAR is subject to the maximum row size and the character set used. For example, utf8 characters can require up to three bytes per character, so a VARCHAR column that uses the utf8 character set can be declared to be a maximum of 21,844 characters.
For the ColumnStore engine, M represents the maximum column length in bytes.
MariaDB stores VARCHAR values as a one-byte or two-byte length prefix plus data. The length prefix indicates the number of bytes in the value. A VARCHAR column uses one length byte if values require no more than 255 bytes, two length bytes if values may require more than 255 bytes.
MariaDB follows the standard SQL specification, and does not remove trailing spaces from VARCHAR values.
VARCHAR(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 VARCHAR(0).
VARCHAR is shorthand for CHARACTER VARYING. NATIONAL VARCHAR is the standard SQL way to define that a VARCHAR column should use some predefined character set. MariaDB uses utf8 as this predefined character set, as does MySQL. NVARCHAR is shorthand for NATIONAL VARCHAR.
For MariaDB, a number of NO PAD collations are available.
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.
The following are synonyms for VARCHAR:
The following are equivalent:
VARCHAR(30) CHARACTER SET utf8
NATIONAL VARCHAR(30)
NVARCHAR(30)
NCHAR VARCHAR(30)
NATIONAL CHARACTER VARYING(30)
NATIONAL CHAR VARYING(30)CREATE TABLE strtest (v VARCHAR(10));
INSERT INTO strtest VALUES('Maria ');
SELECT v='Maria',v='Maria ' FROM strtest;
+-----------+--------------+
| v='Maria' | v='Maria ' |
+-----------+--------------+
| 1 | 1 |
+-----------+--------------+
SELECT v LIKE 'Maria',v LIKE 'Maria ' FROM strtest;
+----------------+-------------------+
| v LIKE 'Maria' | v LIKE 'Maria ' |
+----------------+-------------------+
| 0 | 1 |
+----------------+-------------------+For our example of VARCHAR, we picked a maximum size that avoids overflowing the maximum row size (65535). Keep in mind that a multi-byte character set would need more space in the row than a single-byte character set. We also avoid the auto-conversion of a VARCHAR into a TEXT, MEDIUMTEXT, or LONGTEXT that can happen when STRICT_TRANS_TABLES is not set in the SQL_MODE.
The example:
CREATE TABLE varchar_example (
description VARCHAR(20),
example VARCHAR(65511)
) DEFAULT CHARSET=latin1; -- One byte per char makes the examples clearerINSERT INTO varchar_example VALUES
('Normal foo', 'foo'),
('Trailing spaces foo', 'foo '),
('NULLed', NULL),
('Empty', ''),
('Maximum', RPAD('', 65511, 'x'));SELECT description, LENGTH(example) AS length
FROM varchar_example;+---------------------+--------+
| description | length |
+---------------------+--------+
| Normal foo | 3 |
| Trailing spaces foo | 9 |
| NULLed | NULL |
| Empty | 0 |
| Maximum | 65511 |
+---------------------+--------+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 VARCHAR:
TRUNCATE varchar_example;
INSERT INTO varchar_example VALUES
('Overflow', RPAD('', 65512, 'x'));ERROR 1406 (22001): Data too long for column 'example' at row 1Depending on whether or not strict sql mode is set, you will either get a warning or an error if you try to insert a string that is too long into a VARCHAR column. If the extra characters are spaces, the spaces that can't fit will be removed and you will always get a warning, regardless of the sql mode setting.
VARCHAR columns can be fully indexed. TEXT columns can only be indexed over a specified length.
In Oracle mode, VARCHAR2 is a synonym.
Internally, the full length of the VARCHAR column is allocated inside each TABLE objects record[] structure. As there are three such buffers, each open table will allocate 3 times max-length-to-store-varchar bytes of memory.
TEXT and BLOB columns are stored with a pointer (4 or 8 bytes) + a 1-4 bytes length. The TEXT data is only stored once. This means that internally TEXT uses less memory for each open table but instead has the additional overhead that each TEXT object needs to be allocated and freed for each row access (with some caching in between).
This page is licensed: GPLv2, originally from fill_help_tables.sql
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