mysql唯一索引的一个小常识--Duplicate entry 'XXX' for key 'XXX'

概述

之前一直有个小误区，我以为mysql的唯一索引肯定是区分大小写的，然而实际上utf8字符集下，默认排序规则utf8_general_ci 情况下，是不区分大小写的。而在排序规则utf8_bin下是区分大小写的，这就有可能出现以下情况：之前字段是varchar  binary类型，即排序规则为utf8_bin，后来将该字段改回varchar的话，就会导致唯一键冲突错误

测试

mysql> show create table xx\G
*************************** 1. row ***************************
       Table: xx
Create Table: CREATE TABLE `xx` (
  `id` int(11) DEFAULT NULL,
  `name` varchar(100) NOT NULL,
  `name1` varchar(100) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL,
  PRIMARY KEY (`name`),
  UNIQUE KEY `name1` (`name1`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

mysql> select * from xx;
Empty set (0.00 sec)

mysql> insert into xx(id,name,name1) values(1,'jiang','jiang');
Query OK, 1 row affected (0.00 sec)

mysql> insert into xx(id,name,name1) values(1,'Jiang','jason');
ERROR 1062 (23000): Duplicate entry 'Jiang' for key 'PRIMARY'
mysql> insert into xx(id,name,name1) values(1,'Jiang1','jason');
Query OK, 1 row affected (0.00 sec)

mysql> insert into xx(id,name,name1) values(1,'Jiang2','Jason');
Query OK, 1 row affected (0.00 sec)

mysql> select * from xx;
+------+--------+-------+
| id   | name   | name1 |
+------+--------+-------+
|    1 | jiang  | jiang |
|    1 | Jiang1 | jason |
|    1 | Jiang2 | Jason |
+------+--------+-------+
3 rows in set (0.00 sec)


mysql> alter table xx modify name1 varchar(100);
ERROR 1062 (23000): Duplicate entry 'Jason' for key 'name1'