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4-2 MySQL-如何处理复杂的重复数据删除
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4-1 MySQL-利用主键删除重复数据
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3-2 如何使用SQL语句建立特殊需求的序列号
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3-1 MySQL-在数据库中立唯一序列号的常用方法
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2-8 MySQL-使用序列化表的方法实现列转行
select user_name, case when c.id = 1 then 'arms' when c.id = 2 then 'clothing' when c.id = 3 then 'shoe' end as equipment ,coalesce(case when c.id = 1 then arms end ,case when c.id = 2 then clothing end ,case when c.id = 3 then shoe end) as eq_name from user1 a join user1_equipment b on a.id = b.user_id cross join tb_sequence c where c.id <= 3 order by user_name;
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2-7 MySQL-使用UNION的方法实现列转行
select user_name,'arms' as equipment,arms from user1 a join user1_equipment b on a.id=b.user_id union all select user_name,'clothing' as equipment,clothing from user1 a join user1_equipment b on a.id=b.user_id union all select user_name,'shoe' as equipment,shoe from user1 a join user1_equipment b on a.id=b.user_id order by user_name;
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2-6 MySQL-使用序列化表的方法实现行转列
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2-5 MySQL-单列转多行场景介绍
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2-4 MySQL-使用CASE方法实现行转列
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2-3 MySQL-使用自连接的方法实现行转列
select a.`user_name`,kills from user1 a join user_kills b on a.id=b.user_id; select a.`user_name`,sum(kills) from user1 a join user_kills b on a.id=b.user_id group by a.user_name; select a.`user_name`,sum(kills) from user1 a join user_kills b on a.id=b.user_id group by a.user_name='孙悟空'; select * from ( select sum(kills) as '孙悟空' from user1 a join user_kills b on a.id=b.user_id group by a.user_name='孙悟空'; ) a cross join ( select sum(kills) as '猪八戒' from user1 a join user_kills b on a.id=b.user_id group by a.user_name='猪八戒'; ) b cross join ( select sum(kills) as '沙僧' from user1 a join user_kills b on a.id=b.user_id group by a.user_name='沙僧'; ) c
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2-2 MySQL-需求分析
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2-1 MySQL-行转列场景介绍
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1-1 MySQL开发技巧（二）课程介绍
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多条sql连接查询UNION ALL 操作用于结合两个或更多 SELECT 语句的结果集，包括所有匹配的行，甚至包括重复的行。这与 UNION 不同，因为 UNION 会自动删除重复的行。
满足条件：
1、两个select查询的列的数量必须相同。
2、每个列的数据类型需要相似。
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汇总显示1234
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报表统计1234
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