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UNION 与UNION ALL 的区别?
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#IAJL14
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Opened this issue
2024-08-13 10:11
<h4><strong>返回结果</strong></h4><p>union all是直接连接,取到得是所有值,记录可能有重复;</p><p>union 是取唯一值,记录没有重复。</p><h4><strong>排序</strong></h4><p>union all只是简单的将两个结果合并后就返回。</p><p>union将会按照字段的顺序进行全量排序;</p><h4><strong>效率</strong></h4><p>union all 要比union快很多,所以,如果可以确认合并的两个结果集中,不包含重复数据且不需要排序时的话,那么就使用union all。</p>
<h4><strong>返回结果</strong></h4><p>union all是直接连接,取到得是所有值,记录可能有重复;</p><p>union 是取唯一值,记录没有重复。</p><h4><strong>排序</strong></h4><p>union all只是简单的将两个结果合并后就返回。</p><p>union将会按照字段的顺序进行全量排序;</p><h4><strong>效率</strong></h4><p>union all 要比union快很多,所以,如果可以确认合并的两个结果集中,不包含重复数据且不需要排序时的话,那么就使用union all。</p>
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