通常用UUID做唯一标识,需要在数据库中进行存储。
UUID的格式
代码如下:
String string = UUID.randomUUID().toString(); System.out.println(“uuid:” + string); uuid:05ba463f-1dab-471f-81c7-58e0b06f35f0
数据库中直接存储UUID的坏处:
完全‘随机'的字符串,例如由MD5()、SHA1()、UUID()产生的。它们产生的每一个新值都会被任意地保存在很大的空间范围内,这会减慢INSERT及一些SELECT查询。1)它们会减慢INSERT查询,因为插入的值会被随机地放入索引中。这会导致分页、随机磁盘访问及聚集存储引擎上的聚集索引碎片。2)它们会减慢SELECT查询,因为逻辑上相邻的行会分布在磁盘和内存中的各个地方。3)随机值导致缓存对所有类型的查询性能都很差,因为它们会使缓存赖以工作的访问局部性失效。如果整个数据集都变得同样“热”的时候,那么把特定部分的数据缓存到内存中就没有任何的优势了。并且如果工作集不能被装入内存中,缓存就会进行很多刷写的工作,并且会导致很多缓存未命中。
如果保存UUID值,就应该移除其中的短横线,更好的办法是使用UHEX()把UUID值转化为16字节的数字,并把它保存在BINARY(16)列中。
代码如下:
DELIMITER $$ CREATE FUNCTION `GuidToBinary`( $Data VARCHAR(36) ) RETURNS binary(16) BEGIN DECLARE $Result BINARY(16) DEFAULT NULL; IF $Data IS NOT NULL THEN SET $Data = REPLACE($Data,'-',”); SET $Result = CONCAT(UNHEX(SUBSTRING($Data,7,2)),UNHEX(SUBSTRING($Data,5,2)),UNHEX(SUBSTRING($Data,3,2)), UNHEX(SUBSTRING($Data,1,2)), UNHEX(SUBSTRING($Data,11,2)),UNHEX(SUBSTRING($Data,9,2)),UNHEX(SUBSTRING($Data,15,2)) , UNHEX(SUBSTRING($Data,13,2)), UNHEX(SUBSTRING($Data,17,16))); END IF; RETURN $Result; END $$ CREATE FUNCTION `ToGuid`( $Data BINARY(16) ) RETURNS char(36) CHARSET utf8 BEGIN DECLARE $Result CHAR(36) DEFAULT NULL; IF $Data IS NOT NULL THEN SET $Result = CONCAT(HEX(SUBSTRING($Data,4,1)),HEX(SUBSTRING($Data,3,1)),HEX(SUBSTRING($Data,2,1)), HEX(SUBSTRING($Data,1,1)) , ‘-', HEX(SUBSTRING($Data,6,1)),HEX(SUBSTRING($Data,5,1)),'-', HEX(SUBSTRING($Data,8,1)) , HEX(SUBSTRING($Data,7,1)),'-', HEX(SUBSTRING($Data,9,2)),'-',HEX(SUBSTRING($Data,11,6))); END IF; RETURN $Result; END
代码如下:
CREATE FUNCTION `UUIDTOBIN`() RETURNS binary(16) BEGIN DECLARE my_uuid char(36); SET my_uuid = UUID(); RETURN CONCAT(UNHEX(LEFT(my_uuid,8)),UNHEX(MID(my_uuid,10,4)),UNHEX(MID(my_uuid,15,4)),UNHEX(MID(my_uuid,20,4)),UNHEX(RIGHT(my_uuid,12))); END CREATE FUNCTION `BINTOUUID`(UUID BINARY(16)) RETURNS char(36) BEGIN RETURN CONCAT(HEX(LEFT(uuid,4)),'-', HEX(MID(uuid,5,2)),'-', HEX(MID(uuid,7,2)),'-',HEX(MID(uuid,9,2)),'-',HEX(RIGHT(uuid,6))); END

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MySQL is suitable for beginners to learn database skills. 1. Install MySQL server and client tools. 2. Understand basic SQL queries, such as SELECT. 3. Master data operations: create tables, insert, update, and delete data. 4. Learn advanced skills: subquery and window functions. 5. Debugging and optimization: Check syntax, use indexes, avoid SELECT*, and use LIMIT.

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