http://dev.mysql.com/doc/refman/5.5/en/charset-unicode-utf8mb4.html
utf8mb4比utf8支持更多的字符????
10.1.10.6 The utf8mb4 Character Set (4-Byte UTF-8 Unicode Encoding)
The character set named utf8
uses a maximum of three bytes per character and contains only BMP characters. As of MySQL 5.5.3, the utf8mb4
character set uses a maximum of four bytes per character supports supplemental characters:
-
For a BMP character,
utf8
andutf8mb4
have identical storage characteristics: same code values, same encoding, same length. -
For a supplementary character,
utf8
cannot store the character at all, whileutf8mb4
requires four bytes to store it. Sinceutf8
cannot store the character at all, you do not have any supplementary characters inutf8
columns and you need not worry about converting characters or losing data when upgradingutf8
data from older versions of MySQL.
utf8mb4
is a superset of utf8
, so for an operation such as the following concatenation, the result has character set utf8mb4
and the collation of utf8mb4_col
:
SELECT CONCAT(utf8_col, utf8mb4_col);
Similarly, the following comparison in the WHERE
clause works according to the collation of utf8mb4_col
:
SELECT * FROM utf8_tbl, utf8mb4_tbl WHERE utf8_tbl.utf8_col = utf8mb4_tbl.utf8mb4_col;
Tip: To save space with utf8mb4
, use VARCHAR
instead of CHAR
. Otherwise, MySQL must reserve four bytes for each character in a CHAR CHARACTER SET utf8mb4
column because that is the maximum possible length. For example, MySQL must reserve 40 bytes for a CHAR(10) CHARACTER SET utf8mb4
column