utf8mb4的大小写敏感性测试及其修改方法

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_test\G;
mysql> show create table tbl_test\G;
*************************** 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'