什么是事务

什么是事务呢？简单的说，

 就是确保"同时成功则成功，任何一个失败则失败"的一种机制。一个事务往往包括三种动作行为：开始事务（Begin Transaction），提交事务（Commit）和回滚（Rollback）。从开始事务到提交事务过程中所发生的一切数据库修改要么同时成功（被Commit，固化在数据库中），要么一个失败，大家同时回复原有状态（Rollback，数据库回复到事务开始时的状态）。

具体的英语解释：

Transaction Overview

A database transaction is a larger unit that frames multiple SQL statements. A transaction ensures that the action of the framed statements is atomic with respect to recovery.

A SQL Modification Statement has limited effect. A given statement can only directly modify the contents of a single table (Referential Integrity effects may cause indirect modification of other tables.) The upshot is that operations which require modification of several tables must involve multiple modification statements. A classic example is a bank operation that transfers funds from one type of account to another, requiring updates to 2 tables. Transactions provide a way to group these multiple statements in one atomic unit.

In SQL92, there is no BEGIN TRANSACTION statement. A transaction begins with the execution of a SQL-Data statement when there is no current transaction. All subsequent SQL-Data statements until COMMIT or ROLLBACK become part of the transaction. Execution of a COMMIT Statement or ROLLBACK Statement completes the current transaction. A subsequent SQL-Data statement starts a new transaction.

In terms of direct effect on the database, it is the SQL Modification Statements that are the main consideration since they change data. The total set of changes to the database by the modification statements in a transaction are treated as an atomic unit through the actions of the transaction. The set of changes either:

• Is made fully persistent in the database through the action of the COMMIT Statement, or
• Has no persistent effect whatever on the database, through:
  • the action of the ROLLBACK Statement,
  • abnormal termination of the client requesting the transaction, or
  • abnormal termination of the transaction by the DBMS. This may be an action by the system (deadlock resolution) or by an administrative agent, or it may be an abnormal termination of the DBMS itself. In the latter case, the DBMS must roll back any active transactions during recovery.

The DBMS must ensure that the effect of a transaction is not partial. All changes in a transaction must be made persistent, or no changes from the transaction must be made persistent.

我的理解：

最基本的， 在数据库处理DML （Update Insert Delete） 时，会在数据库形成过程任务需要执行 commit 或 因为异常存在而取消该过程的执行 这种过程加判断的结果 整套数据库运行的机制为事务 。