utf8mb4的大小写敏感性测试及其修改方法
utf8mb4_ unicode_ ci 与 utf8mb4_ general_ ci 如何选择
字符除了需要存储，还需要排序或比较大小，涉及到与编码字符集对应的 排序字符集（collation）。ut8mb4对应的排序字符集常用的有 utf8mb4_unicode_ci 、 utf8mb4_general_ci ，到底采用哪个在 stackoverflow 上有个讨论， What’s the difference between utf8_general_ci and utf8_unicode_ci
主要从排序准确性和性能两方面看：
准确性
utf8mb4_unicode_ci 是基于标准的Unicode来排序和比较，能够在各种语言之间精确排序
utf8mb4_general_ci 没有实现Unicode排序规则，在遇到某些特殊语言或字符是，排序结果可能不是所期望的。
但是在绝大多数情况下，这种特殊字符的顺序一定要那么精确吗。比如Unicode把 ? 、 ? 当成 ss 和 OE 来看；而general会把它们当成 s 、 e ，再如 àá??ā? 各自都与 A 相等。
性能
utf8mb4_general_ci 在比较和排序的时候更快
utf8mb4_unicode_ci 在特殊情况下，Unicode排序规则为了能够处理特殊字符的情况，实现了略微复杂的排序算法。
但是在绝大多数情况下，不会发生此类复杂比较。general理论上比Unicode可能快些，但相比现在的CPU来说，它远远不足以成为考虑性能的因素，索引涉及、SQL设计才是。 我个人推荐是 utf8mb4_unicode_ci ，将来 8.0 里也极有可能使用变为默认的规则。
# 测试utf8mb4的大小写敏感性及其修改方法 -- 以下是utf8mb4不区分大小写 # 修改数据库: ALTER DATABASE database_name CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci; # 修改表: ALTER TABLE table_name CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci; # 修改表字段: ALTER TABLE table_name CHANGE column_name column_name VARCHAR(191) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL; -- 以下是utf8mb4区分大小写 # 修改数据库: ALTER DATABASE database_name CHARACTER SET = utf8mb4 COLLATE = utf8mb4_bin; # 修改表: ALTER TABLE table_name CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_bin; # 修改表字段: ALTER TABLE table_name CHANGE column_name column_name VARCHAR(191) CHARACTER SET utf8mb4 COLLATE utf8mb4_bin NOT NULL ;
-- 1、删除库 drop database if exists db2020; mysql> drop database if exists db2020; Query OK, 0 rows affected, 1 warning (0.00 sec) -- 2、创建字符集为utf8的库 create database db2020 DEFAULT CHARACTER SET utf8mb4; mysql> create database db2020 DEFAULT CHARACTER SET utf8mb4; Query OK, 1 row affected (0.00 sec) -- 3、查看建库语句 show create database db2020; mysql> show create database db2020; +----------+--------------------------------------------------------------------+ | Database | Create Database | +----------+--------------------------------------------------------------------+ | db2020 | CREATE DATABASE `db2020` /*!40100 DEFAULT CHARACTER SET utf8mb4 */ | +----------+--------------------------------------------------------------------+ 1 row in set (0.00 sec) -- 4、创建测试表和数据 use db2020; -- drop table if exists tbl_test ; create table tbl_test ( id bigint(20) NOT NULL AUTO_INCREMENT, name varchar(20) NOT NULL, PRIMARY KEY (id), KEY idx_name (name) ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 ; -- 5、查看创建表的语句 -- use db2020; show create table tbl_testG; mysql> show create table tbl_testG; *************************** 1. row *************************** Table: tbl_test Create Table: CREATE TABLE `tbl_test` ( `id` bigint(20) NOT NULL AUTO_INCREMENT, `name` varchar(20) NOT NULL, PRIMARY KEY (`id`), KEY `idx_name` (`name`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 1 row in set (0.05 sec) ERROR: No query specified -- 6、查看默认字符集 -- 方法1、show variables like '%character%'; mysql> show variables like '%character%'; +--------------------------+----------------------------------------------------------------+ | Variable_name | Value | +--------------------------+----------------------------------------------------------------+ | character_set_client | utf8 | | character_set_connection | utf8 | | character_set_database | utf8mb4 | | character_set_filesystem | binary | | character_set_results | utf8 | | character_set_server | utf8mb4 | | character_set_system | utf8 | | character_sets_dir | /opt/mysql/mysql-5.6.43-linux-glibc2.12-x86_64/share/charsets/ | +--------------------------+----------------------------------------------------------------+ 8 rows in set (0.00 sec) -- 方法2、show variables like 'collation%'; mysql> show variables like 'collation%'; +----------------------+--------------------+ | Variable_name | Value | +----------------------+--------------------+ | collation_connection | utf8_general_ci | | collation_database | utf8mb4_general_ci | | collation_server | utf8mb4_general_ci | +----------------------+--------------------+ 3 rows in set (0.00 sec) -- 8、查看默认校对规则 show collation like 'utf8mb4%'; mysql> show collation like 'utf8mb4%'; +------------------------+---------+-----+---------+----------+---------+ | Collation | Charset | Id | Default | Compiled | Sortlen | +------------------------+---------+-----+---------+----------+---------+ | utf8mb4_general_ci | utf8mb4 | 45 | Yes | Yes | 1 | | utf8mb4_bin | utf8mb4 | 46 | | Yes | 1 | | utf8mb4_unicode_ci | utf8mb4 | 224 | | Yes | 8 | ...... ...... +------------------------+---------+-----+---------+----------+---------+ 26 rows in set (0.52 sec) -- 9、插入测试数据 -- use db2020; insert into tbl_test(name) values('aaa'); insert into tbl_test(name) values('bbb'); insert into tbl_test(name) values('AAA'); insert into tbl_test(name) values('BBB'); mysql> select * from tbl_test; +----+------+ | id | name | +----+------+ | 1 | aaa | | 3 | AAA | | 2 | bbb | | 4 | BBB | +----+------+ 4 rows in set (0.08 sec) mysql> select * from tbl_test where name='aaa'; +----+------+ | id | name | +----+------+ | 1 | aaa | | 3 | AAA | +----+------+ 2 rows in set (0.04 sec) use db2020; insert into tbl_test(name) values('aaa'); insert into tbl_test(name) values('bbb'); insert into tbl_test(name) values('AAA'); insert into tbl_test(name) values('BBB'); mysql> select * from tbl_test; +----+------+ | id | name | +----+------+ | 1 | aaa | | 3 | AAA | | 2 | bbb | | 4 | BBB | +----+------+ 4 rows in set (0.00 sec) mysql> select * from tbl_test where name='aaa'; +----+------+ | id | name | +----+------+ | 1 | aaa | | 3 | AAA | +----+------+ 2 rows in set (0.00 sec) -- 10、默认情况下,不区分大小写,修改成大小写敏感 -- alter database db2020 character set=utf8mb4; alter database db2020 character set=utf8mb4 collate=utf8mb4_bin; -- alter table tbl_test convert to character set utf8mb4 ; alter table tbl_test convert to character set utf8mb4 collate utf8mb4_bin; -- 只修改这个即可实现区分大小写 -- alter table tbl_test change name name varchar(20) character set utf8mb4 collate utf8mb4_general_ci not null; -- alter table tbl_test modify name varchar(20) character set utf8mb4 collate utf8mb4_general_ci not null; alter table tbl_test change name name varchar(20) character set utf8mb4 collate utf8mb4_bin not null; alter table tbl_test modify name varchar(20) character set utf8mb4 collate utf8mb4_bin not null; mysql> alter database db2020 character set=utf8mb4 collate=utf8mb4_bin; Query OK, 1 row affected (0.00 sec) mysql> show create database db2020; +----------+----------------------------------------------------------------------------------------+ | Database | Create Database | +----------+----------------------------------------------------------------------------------------+ | db2020 | CREATE DATABASE `db2020` /*!40100 DEFAULT CHARACTER SET utf8mb4 COLLATE utf8mb4_bin */ | +----------+----------------------------------------------------------------------------------------+ 1 row in set (0.00 sec) mysql> select * from tbl_test where name='aaa'; +----+------+ | id | name | +----+------+ | 1 | aaa | | 3 | AAA | +----+------+ 2 rows in set (0.00 sec) -- 此时只修改库级别的还不行，仍然还需要修改表级别的 mysql> alter table tbl_test convert to character set utf8mb4 collate utf8mb4_bin; Query OK, 4 rows affected (0.08 sec) Records: 4 Duplicates: 0 Warnings: 0 mysql> select * from tbl_test where name='aaa'; +----+------+ | id | name | +----+------+ | 1 | aaa | +----+------+ 1 row in set (0.00 sec) -- 附录 修改MySQL配置文件,新增如下参数： [client] default-character-set = utf8mb4 [mysql] default-character-set = utf8mb4 [mysqld] character-set-client-handshake = FALSE character-set-server = utf8mb4 collation-server = utf8mb4_unicode_ci init_connect='SET NAMES utf8mb4'