MySQL 的唯一索引(unique index)很好理解,就是这个索引的值是唯一的.唯一索引的相关文档和概念不多,只要记住"唯一"这个核心概念就行
这是MySQL Unique Indexes的官方原文
Unique IndexesA UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. If you specify a prefix value for a column in a UNIQUE index, the column values must be unique within the prefix length. A UNIQUE index permits multiple NULL values for columns that can contain NULL.
唯一索引创建了一个约束,它规定索引中的全部的值必须是不同的唯一的.如果尝试添加一个与现有行索引值相等新行,就会发生错误.部分索引可以设置为唯一索引,但是被设置为唯一的部分索引值也必须是唯一的.
A UNIQUE index permits multiple NULL values for columns that can contain NULL.
这个其实是以一个面试点,被设置为唯一索引的列的值是允许有null值的,并且这个列中允许有多个重复的null值.
代码验证:
mysql> create table unique_index_test(
-> id int primary key,
-> name varchar(16),
-> unique index uk_name (name)
-> );
Query OK, 0 rows affected (0.03 sec)
mysql> desc unique_index_test;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int | NO | PRI | NULL | |
| name | varchar(16) | YES | UNI | NULL | |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
mysql> show index from unique_index_test;
+-------------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+-------------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| unique_index_test | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | | YES | NULL |
| unique_index_test | 0 | uk_name | 1 | name | A | 0 | NULL | NULL | YES | BTREE | | | YES | NULL |
+-------------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
2 rows in set (0.01 sec)
我们验证下唯一索引是否可以允许值为null,并且可以有多个null
mysql> insert into unique_index_test values (1,"thinktik1");
Query OK, 1 row affected (0.00 sec)
mysql> insert into unique_index_test values (2,"thinktik2");
Query OK, 1 row affected (0.01 sec)
mysql> insert into unique_index_test values (3,null);
Query OK, 1 row affected (0.00 sec)
mysql> insert into unique_index_test values (4,null);
Query OK, 1 row affected (0.01 sec)
mysql> select * from unique_index_test;
+----+-----------+
| id | name |
+----+-----------+
| 3 | NULL |
| 4 | NULL |
| 1 | thinktik1 |
| 2 | thinktik2 |
+----+-----------+
4 rows in set (0.00 sec)
mysql> insert into unique_index_test values (5,"thinktik1");
ERROR 1062 (23000): Duplicate entry 'thinktik1' for key 'unique_index_test.uk_name'
结论是: 唯一索引可以允许值为null,并且可以有多个null
同时唯一索引在使用的时候有一些附加约束: PRIMARY KEY and UNIQUE Index Constraints
Normally, errors occur for data-change statements (such as INSERT or UPDATE) that would violate primary-key, unique-key, or foreign-key constraints. If you are using a transactional storage engine such as InnoDB, MySQL automatically rolls back the statement. If you are using a nontransactional storage engine, MySQL stops processing the statement at the row for which the error occurred and leaves any remaining rows unprocessed.
通常，违反主键、唯一键或外键约束的数据更改语句(如INSERT或UPDATE)会发生错误。如果你使用的是事务性存储引擎，比如InnoDB, MySQL会自动回滚语句。如果您使用的是非事务性存储引擎，MySQL将停止处理发生错误的行上的语句，并保留任何剩余的行不处理.
我个人认为还是InnoDB这类支持事务的MySQL引擎好,MySQL 8已经默认使用InnoDB作为引擎了,比MyISAM综合性能更好,功能也更全面.