출처 : http://dev.mysql.com/doc/refman/5.0/en/char.html
칼럼의 값이 일정한지, 가변적인지에 대해서 선택을 하면 된다.
만약, 코드의 값이 2byte(01,AA,Ad,a4,z9,98,5g...)로 일정할때는 char을 써야한다.
가령, 2byte의 값이 char은 2byte지만 varchar은 3byte를 사용하게 된다.
그러나, 주소처럼 가변적인 경우에는 varchar을 써야한다고 권고하고있다.
그 이유는 varchar의 경우 100byte로 정의 해도 80byte만 삽입되었다면 19byte가 남기때문이다.
char 경우에는 100byte 모두 사용해 낭비 하기때문이다.
모든 데이터베이스(Oracle,MySQL,MS-SQL...)의 공통 사항임.
The CHAR and VARCHAR 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 effective maximum length of a VARCHAR in MySQL 5.0.3 and later is subject to the maximum row size (65,535 bytes, which is shared among all columns) and the character set used.
In contrast to CHAR, VARCHAR values are stored as a one-byte or two-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 5.1.6, “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. 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.
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 (assuming that the column uses a single-byte character set such as latin1).
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.
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)
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 more information about MySQL character sets and collations, see Section 9.1, “Character Set Support”.
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.