left join on去重

在使用left join时进行去重有多种方法可供选择。其中一种方法是使用关键字DISTINCT，例如SELECT DISTINCT a.id FROM a LEFT JOIN c ON a.id=c.a_id where c.b_id IS NOT NULL，这样可以获得a.id字段的唯一值结果，其他字段需要在内存中再次查询拼装，适用于返回单个特定值去重或返回去重后结果数量。另一种方法是使用GROUP BY进行分组，例如SELECT a.* FROM a LEFT JOIN c ON a.id=c.a_id where c.b_id IS NOT NULL GROUP BY a.id，这种方法在left join的结果基础上进行分组，直接去重，支持完美分页和查询结果，但由于GROUP BY随机返回重复行，表c中的数据无法使用，必须再次查询后内存中拼装。请根据实际场景选择适合的方法。<span class="em">1</span><span class="em">2</span><span class="em">3</span>
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- *1* *3* [多表连接查询数据拼接、left join去重问题](https://blog.csdn.net/qq_26012495/article/details/108923936)[target="_blank" data-report-click={"spm":"1018.2226.3001.9630","extra":{"utm_source":"vip_chatgpt_common_search_pc_result","utm_medium":"distribute.pc_search_result.none-task-cask-2~all~insert_cask~default-1-null.142^v92^chatsearchT0_1"}}] [.reference_item style="max-width: 50%"]
- *2* [从简单实例来看 left join 如何去重](https://blog.csdn.net/locahuang/article/details/125337397)[target="_blank" data-report-click={"spm":"1018.2226.3001.9630","extra":{"utm_source":"vip_chatgpt_common_search_pc_result","utm_medium":"distribute.pc_search_result.none-task-cask-2~all~insert_cask~default-1-null.142^v92^chatsearchT0_1"}}] [.reference_item style="max-width: 50%"]
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