mysql字符类型大小写敏感的讨论

mysql字符类型默认是不区分大小写的，即select * from t where name='AAA'与='aaa'没区别，以下是测试的例子

(root@localhost)[hello]> create table test1(id int, name varchar(10));
(root@localhost)[hello]> insert into test1 values(1,'aaa'),(2,'AAA'),(3,'bbb'),(4,'BbB');
(root@localhost)[hello]> select * from test1;
+------+------+
| id   | name |
+------+------+
|    1 | aaa  |
|    2 | AAA  |
|    3 | bbb  |
|    4 | BbB  |
+------+------+

(root@localhost)[hello]> select * from test1 where name = 'AAA';
+------+------+
| id   | name |
+------+------+
|    1 | aaa  |
|    2 | AAA  |
+------+------+

(root@localhost)[hello]> select * from test1 where name = 'aaa';
+------+------+
| id   | name |
+------+------+
|    1 | aaa  |
|    2 | AAA  |
+------+------+

可以看到此时where条件后面的'AAA'与'aaa'，查出来的结果没啥区别。

如果只想找出'AAA'的可以有以下几种办法
1.在sql中加入binary关键字

(root@localhost)[hello]> select * from test1 where binary name = 'AAA';
+------+------+
| id   | name |
+------+------+
|    2 | AAA  |
+------+------+

2.修改列的定义

先查看原始表的定义

(root@localhost)[hello]> show create table test1\G
*************************** 1. row ***************************
       Table: test1
Create Table: CREATE TABLE `test1` (
  `id` int(11) DEFAULT NULL,
  `name` varchar(10) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4

修改表test1的name列

alter table test1 modify column name varchar(10) character set utf8mb4 collate utf8mb4_bin default null;
collate utf8mb4_bin表示where过滤或者order by排序区分大小写

此时查看test1的定义

(root@localhost)[hello]> show create table test1\G
*************************** 1. row ***************************
       Table: test1
Create Table: CREATE TABLE `test1` (
  `id` int(11) DEFAULT NULL,
  `name` varchar(10) CHARACTER SET utf8mb4 COLLATE utf8mb4_bin DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4

接着再执行查询语句

(root@localhost)[hello]> select * from test1 where name='AAA';
+------+------+
| id   | name |
+------+------+
|    2 | AAA  |
+------+------+

下面再创建一张test2表，就会发现上面修改列的语句其实相当于在创建表时varchar后面跟binary

(root@localhost)[hello]> create table test2(id int, name varchar(10) binary);
(root@localhost)[hello]> show create table test2\G
*************************** 1. row ***************************
       Table: test2
Create Table: CREATE TABLE `test2` (
  `id` int(11) DEFAULT NULL,
  `name` varchar(10) CHARACTER SET utf8mb4 COLLATE utf8mb4_bin DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4

下面介绍如何设置字符大小写敏感

数据库级别设置字符大小写敏感
创建
create database <db_name> default character set utf8mb4 collate utf8mb4_bin;
修改
alter database <db_name> default character set utf8mb4 collate utf8mb4_bin;

表级别设置字符大小写敏感
创建
create table <tb_name> (
......
) engine=innodb default charset=utf8mb4 collate=utf8mb4_bin;
修改
alter table <tb_name> engine=innodb default charset=utf8mb4 collate=utf8mb4_bin;

列级别设置字符大小写敏感
创建
create table <tb_name> (
`field1` varchar(10) character set utf8mb4 collate utf8mb4_bin,
......
)
修改
alter table <tb_name> modify column `field1` varchar(10) character set utf8mb4 collate utf8mb4_bin default null;

继承关系是列-->表-->库，优先级是列>表>库