UNIQUE，UNIQUE KEY和CONSTRAINT'name'UNIQUE有什么区别？

I have a basic 'users' table I want to create in MySQL.

我有一个基本的“用户”表，我想在MySQL中创建。

I do not want duplicate emails or duplicate usernames appearing in the database.

我不希望在数据库中出现重复的电子邮件或重复的用户名。

What is the best way of preventing this upon table creation?
在创建桌子时防止这种情况的最佳方法是什么？
And what is the difference between the following:
以下是有什么区别的：

1. UNIQUE (username), UNIQUE (email),

1. UNIQUE（用户名），UNIQUE（电子邮件），

2. UNIQUE KEY (username), UNIQUE KEY (email),

2. UNIQUE KEY（用户名），UNIQUE KEY（电子邮件），

3. CONSTRAINT ucons_login UNIQUE (username, email),

3. CONSTRAINT ucons_login UNIQUE（用户名，电子邮件），

I assume some of these are synonymous, yet I've been reading conflicting information online and was seeking confirmation.

我假设其中一些是同义词，但我一直在网上阅读相互矛盾的信息，并正在寻求确认。

I hope someone can assist.

我希望有人可以提供帮助。

The SQL:

SQL：

CREATE TABLE users (
user_id INT UNSIGNED NOT NULL AUTO_INCREMENT,
username VARCHAR(30) NOT NULL,
pass CHAR(40) NOT NULL,
first_name VARCHAR(20) NOT NULL,
last_name VARCHAR(40) NOT NULL,
email VARCHAR(60) NOT NULL,
registration_date DATETIME NOT NULL,
user_level TINYINT(1) UNSIGNED NOT NULL DEFAULT 0,
active CHAR(32),
PRIMARY KEY (user_id),
UNIQUE (username),
UNIQUE (email),
INDEX login (email, pass),
INDEX full_name (last_name, first_name)
) ENGINE=INNODB;

2 个解决方案

#1

1 and 2 are identical - both create two unique indexes, one for each key. #3 only creates one unique index across both keys, so no combination of username and email can be duplicated, but for example, a username could be duplicated as long as a different email was used.

1和2是相同的 - 都创建了两个唯一索引，每个索引一个。＃3仅在两个密钥上创建一个唯一索引，因此不能复制用户名和电子邮件的组合，但是，例如，只要使用不同的电子邮件，就可以复制用户名。

Sounds like you probably want either of the first two. UNIQUE and UNIQUE KEY are equivalent.

听起来你可能想要前两个中的任何一个。 UNIQUE和UNIQUE KEY是等价的。

#2

They are all synonymous as evidenced by syntax documentation:

它们都是同义词，如语法文档所示：

[CONSTRAINT [symbol]] UNIQUE [INDEX|KEY] [index_name] [index_type] (index_col_name,...) [index_option]

[CONSTRAINT [symbol]] UNIQUE [INDEX | KEY] [index_name] [index_type]（index_col_name，...）[index_option]

[] in this notation (Wirth's notation) denote optional elements

[]符号（Wirth的符号）表示可选元素

#1