utf8mb4的大小寫敏感性測試及其修改方法
utf8mb4_ unicode_ ci 與 utf8mb4_ general_ ci 如何選擇
字符除了需要存儲，還需要排序或比較大小，涉及到與編碼字符集對應的 排序字符集（collation）。ut8mb4對應的排序字符集常用的有 utf8mb4_unicode_ci 、 utf8mb4_general_ci ，到底采用哪個在 stackoverflow 上有個討論， What’s the difference between utf8_general_ci and utf8_unicode_ci
主要從排序准確性和性能兩方面看：
准確性
utf8mb4_unicode_ci 是基於標准的Unicode來排序和比較，能夠在各種語言之間精確排序
utf8mb4_general_ci 沒有實現Unicode排序規則，在遇到某些特殊語言或字符是，排序結果可能不是所期望的。
但是在絕大多數情況下，這種特殊字符的順序一定要那么精確嗎。比如Unicode把 ? 、 ? 當成 ss 和 OE 來看；而general會把它們當成 s 、 e ，再如 àá??ā? 各自都與 A 相等。
性能
utf8mb4_general_ci 在比較和排序的時候更快
utf8mb4_unicode_ci 在特殊情況下，Unicode排序規則為了能夠處理特殊字符的情況，實現了略微復雜的排序算法。
但是在絕大多數情況下，不會發生此類復雜比較。general理論上比Unicode可能快些，但相比現在的CPU來說，它遠遠不足以成為考慮性能的因素，索引涉及、SQL設計才是。 我個人推薦是 utf8mb4_unicode_ci ，將來 8.0 里也極有可能使用變為默認的規則。
# 測試utf8mb4的大小寫敏感性及其修改方法 -- 以下是utf8mb4不區分大小寫 # 修改數據庫: 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區分大小寫 # 修改數據庫: 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、刪除庫 drop database if exists db2020; mysql> drop database if exists db2020; Query OK, 0 rows affected, 1 warning (0.00 sec) -- 2、創建字符集為utf8的庫 create database db2020 DEFAULT CHARACTER SET utf8mb4; mysql> create database db2020 DEFAULT CHARACTER SET utf8mb4; Query OK, 1 row affected (0.00 sec) -- 3、查看建庫語句 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、創建測試表和數據 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、查看創建表的語句 -- 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、查看默認字符集 -- 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、查看默認校對規則 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、插入測試數據 -- 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、默認情況下,不區分大小寫,修改成大小寫敏感 -- 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; -- 只修改這個即可實現區分大小寫 -- 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) -- 此時只修改庫級別的還不行，仍然還需要修改表級別的 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) -- 附錄 修改MySQL配置文件,新增如下參數： [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'