生成唯一ID值的机制不得受事务隔离的约束.这是数据库为每个客户端生成不同值所必需的，这优于SELECT MAX(id)+1 FROM table的窍门，如果两个客户端尝试同时分配新的id值，则会导致争用情况.

The mechanism to generate unique id values must not be subject to transaction isolation. This is required for the database to generate a distinct value for each client, better than the trick of SELECT MAX(id)+1 FROM table, which results in a race condition if two clients try to allocate new id values concurrently.

您不能使用标准SQL查询来模拟此操作(除非您使用表锁或可序列化事务).它必须是数据库引擎中内置的一种机制.

You can't simulate this operation using standard SQL queries (unless you use table locks or serializable transactions). It has to be a mechanism built into the database engine.

直到SQL:2003，ANSI SQL才描述用于生成代理键唯一值的操作.在此之前，没有自动递增色谱柱的标准，因此几乎每个品牌的RDBMS都提供了一些专有的解决方案.自然，它们相差很大，并且无法以简单的，独立于数据库的方式使用它们.

ANSI SQL did not describe an operation to generate unique values for surrogate keys until SQL:2003. Before that, there was no standard for auto-incrementing columns, so nearly every brand of RDBMS provided some proprietary solution. Naturally they vary a lot, and there's no way to use them in a simple, database-independent manner.

• MySQL具有AUTO_INCREMENT列选项，或SERIAL伪数据类型，其等效于BIGINT UNSIGNED AUTO_INCREMENT;
• Microsoft SQL Server具有IDENTITY列选项和NEWSEQUENTIALID()，介于自动增量和GUID之间；
• Oracle有一个SEQUENCE对象；
• PostgreSQL具有SEQUENCE对象或SERIAL伪数据类型，该伪数据类型根据命名约定隐式创建序列对象；
• InterBase/Firebird有一个GENERATOR对象，该对象非常类似于Oracle中的SEQUENCE. Firebird 2.1也支持SEQUENCE;
• SQLite将声明为您的主键的任何整数视为隐式自动递增；
• DB2 UDB几乎包含所有内容:SEQUENCE对象，或者您可以使用"GEN_ID"选项声明列.

• MySQL has the AUTO_INCREMENT column option, or SERIAL pseudo-datatype which is equivalent to BIGINT UNSIGNED AUTO_INCREMENT;
• Microsoft SQL Server has the IDENTITY column option and NEWSEQUENTIALID() which is something between auto-increment and GUID;
• Oracle has a SEQUENCE object;
• PostgreSQL has a SEQUENCE object, or SERIAL pseudo-datatype which implicitly creates a sequence object according to a naming convention;
• InterBase/Firebird has a GENERATOR object which is pretty much like a SEQUENCE in Oracle; Firebird 2.1 supports SEQUENCE too;
• SQLite treats any integer declared as your primary key as implicitly auto-incrementing;
• DB2 UDB has just about everything: SEQUENCE objects, or you can declare columns with the "GEN_ID" option.

所有这些机制都在事务隔离之外运行，以确保并发客户端获得唯一的值.同样，在所有情况下，都可以查询当前会话的最新生成的值 .必须有，因此您可以使用它在子表中插入行.

All these mechanisms operate outside transaction isolation, ensuring that concurrent clients get unique values. Also in all cases there is a way to query the most recently generated value for your current session. There has to be, so you can use it to insert rows in a child table.