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数据库版本:
MySQL 5.6.26

线上某业务表为了区分大小写，使用BINARY关键字，正常来说使用这个关键字是走索引的，测试过程如下：

创建测试表，插入数据：

drop table  if EXISTS student;

CREATE TABLE `student` (
  `id` int(11) PRIMARY key auto_increment,
  `name` varchar(20) DEFAULT NULL,
key `idx_name`(`name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 ; 

insert into `student` ( `id`, `name`) values ( '1', 'michael');
insert into `student` ( `id`, `name`) values ( '2', 'lucy');
insert into `student` ( `id`, `name`) values ( '3', 'nacy');
insert into `student` ( `id`, `name`) values ( '4', 'mike');
insert into `student` ( `id`, `name`) values ( null, 'guo');
insert into `student` ( `id`, `name`) values ( '6', 'Guo');

不加BINARY关键字可以走索引：

mysql> desc select * from student where name = 'guo';
+----+-------------+---------+------+---------------+----------+---------+-------+------+--------------------------+
| id | select_type | table   | type | possible_keys | key      | key_len | ref   | rows | Extra                    |
+----+-------------+---------+------+---------------+----------+---------+-------+------+--------------------------+
| 1  | SIMPLE      | student | ref  | idx_name      | idx_name | 63      | const | 2    | Using where; Using index |
+----+-------------+---------+------+---------------+----------+---------+-------+------+--------------------------+
1 rows in set (0.03 sec)

正常来说BINARY关键字是可以走索引的：

mysql> desc select * from student where BINARY name = 'guo';
+----+-------------+---------+-------+---------------+----------+---------+------+------+--------------------------+
| id | select_type | table   | type  | possible_keys | key      | key_len | ref  | rows | Extra                    |
+----+-------------+---------+-------+---------------+----------+---------+------+------+--------------------------+
| 1  | SIMPLE      | student | index | NULL          | idx_name | 63      | NULL | 6    | Using where; Using index |
+----+-------------+---------+-------+---------------+----------+---------+------+------+--------------------------+
1 rows in set (0.04 sec)

不使用BINARY关键字默认不会区分大小写：

mysql> select * from student where name = 'guo';
+----+------+
| id | name |
+----+------+
| 5  | guo  |
| 6  | Guo  |
+----+------+
2 rows in set (0.03 sec)

mysql> select * from student where name = 'Guo';
+----+------+
| id | name |
+----+------+
| 5  | guo  |
| 6  | Guo  |
+----+------+
2 rows in set (0.03 sec)

使用BINARY关键字可以区分大小写：

mysql>  select * from student where BINARY name = 'guo';
+----+------+
| id | name |
+----+------+
| 5  | guo  |
+----+------+
1 rows in set (0.04 sec)

mysql>  select * from student where BINARY name = 'Guo';
+----+------+
| id | name |
+----+------+
| 6  | Guo  |
+----+------+
1 rows in set (0.03 sec)

mysql>

到这里以上都没问题，但关键在于，业务的表结构大于索引的最大长度即字串长度超过255。

CREATE TABLE `student` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(2000) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `idx_name` (`name`(255))
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8

mysql> desc select * from student where name = 'guo';
+----+-------------+---------+------+---------------+----------+---------+-------+------+-------------+
| id | select_type | table   | type | possible_keys | key      | key_len | ref   | rows | Extra       |
+----+-------------+---------+------+---------------+----------+---------+-------+------+-------------+
| 1  | SIMPLE      | student | ref  | idx_name      | idx_name | 768     | const | 2    | Using where |
+----+-------------+---------+------+---------------+----------+---------+-------+------+-------------+
1 rows in set (0.04 sec)

加上BINARY关键字不再走索引：

mysql> desc select * from student where BINARY name = 'guo';
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
| 1  | SIMPLE      | student | ALL  | NULL          | NULL | NULL    | NULL | 6    | Using where |
+----+-------------+---------+------+---------------+------+---------+------+------+-------------+
1 rows in set (0.05 sec)

mysql>

这时需要在表结构里加上BINARY

mysql>ALTER TABLE student MODIFY COLUMN name VARCHAR(20) BINARY;
Query OK, 6 rows affected (0.06 sec)

数据库会自动转换成COLLATE utf8_bin
collate关键字为校对集，主要是对字符集之间的比较和排序，可以通过 show collation查看所有的校对集

mysql> show create table student\G
*************************** 1. row ***************************
Table       : student
Create Table: CREATE TABLE `student` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(20) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `idx_name` (`name`)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8
1 rows in set (0.39 sec)

mysql> 


mysql> desc select * from student where name = 'guo';
+----+-------------+---------+------+---------------+----------+---------+-------+------+--------------------------+
| id | select_type | table   | type | possible_keys | key      | key_len | ref   | rows | Extra                    |
+----+-------------+---------+------+---------------+----------+---------+-------+------+--------------------------+
| 1  | SIMPLE      | student | ref  | idx_name      | idx_name | 63      | const | 1    | Using where; Using index |
+----+-------------+---------+------+---------------+----------+---------+-------+------+--------------------------+
1 rows in set (0.07 sec)

mysql>

即可区分大小写：

mysql> select * from student where name = 'guo';
+----+------+
| id | name |
+----+------+
| 5  | guo  |
+----+------+
1 rows in set (0.07 sec)

mysql> select * from student where name = 'Guo';
+----+------+
| id | name |
+----+------+
| 6  | Guo  |
+----+------+
1 rows in set (0.06 sec)

mysql>
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