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<div style="text-align:right;">
  <a title="观看本节视频" href="javascript:showVideo();" style="text-decoration:none;">
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<div id="temp-video-cover">
  <div id="temp-video-container">
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        <span class="FUI-button-svg"></span><span class="FUI-button-text">&#10005;</span></div>
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        <video id="temp-video" src="http://video1.grapecity.com.cn/HuoZiGe/Tutorials/v5.0/教学视频/外联数据库并发控制.mp4"
          height="450" controls="controls" onplaying="startplay()">您的浏览器不支持播放此视频。</video>
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        <div>倍速：</div>
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        <div>0.5X</div>
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        <div>1.25X</div>
        <div>1.5X</div>
        <div>2.0X</div>
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<script>
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</script>

1.描述

并发控制指当多个用户同时更新运行时，用于保护数据库完整性的技术。其目的是保证一个用户的操作不会对另一个用户的使用产生不合理的影响。并发机制不正确可能导致脏读、幻读和不可重复读等此类问题。

2.开启并发控制

使用不同数据库开启并发控制的方式不同。

SQL Server数据库

所使用的表需要包含时间戳数据类型的列，例如SQLServer中的timestamp类型。

Oracle数据库

Oracle数据库需要用户做如下设置：

创建一个用于并发控制的Rowversion列，类型为timestamp，并创建一个触发器，在更新时更新对应的Rowversion的信息为当前的日期。

触发器示例如下：

CREATE OR REPLACE TRIGGER trigger_testRowversion
BEFORE UPDATE
   ON TABLE1
  FOR EACH ROW
BEGIN
  :new.rowversion := current_timestamp;
END trigger_testRowversion;

假设用于并发控制的列的名字为ROWVERSION。在活字格中，连接Oracle时，在右边栏设置面板中的“时间戳列”中选择“ROWVERSION”，即可开启并发控制，如图1所示。