列转行另一种场景

列转行技巧： 1.首先我们加一张序列号表，所谓序列号表就是一张有一个自增字段的表，如id 1,2,3,4,5,6...... 2.使用这条sql语句，这句有点整不懂。 SELECT user_name , REPLACE(SUBSTRING(SUBSTRING_INDEX(mobile,',',a.id),CHAR_LENGTH(SUBSTRING_INDEX(mobile,',',a.id-1))+1),',','') AS mobile//根据逗号进行字符串的截取 FROM tb_sequence a CROSS JOIN ( //数据表的连接 select user_name,concat(mobile,',') as mobile,//concat在字符串后面添上逗号 length(mobile)-length(replace(mobile,',',''))+1 as size //显示出逗号的个数 from user1 as b) as b on a.id<=b.size

列转行场景

使用cross join 方式是实现行列转换： 1.统计每一个用户打怪的总数 select sum(kills) as '沙僧' from user1 a left join user_kills b on a.id = b.user_id where a.username='沙僧'; select sum(kills) as '猪八戒' from user1 a left join user_kills b on a.id = b.user_id where a.username='猪八戒'; select sum(kills) as '孙悟空' from user1 a left join user_kills b on a.id = b.user_id where a.username='孙悟空'; 2.使用cross join 进行笛卡尔积连接，将三组数据进行行转列组合 select * from (select sum(kills) as '沙僧' from user1 a left join user_j=kills b on a.id = b.user_id and a.username='沙僧') a cross join (select sum(kills) as '猪八戒' from user1 a left join user_j=kills b on a.id = b.user_id and a.username='猪八戒') b cross join (select sum(kills) as '孙悟空' from user1 a left join user_j=kills b on a.id = b.user_id and a.username='孙悟空') c 总结：这种方法虽然可以转换行列，但是遇到需要组合的数据太多编写起来就比较繁琐，而且查询效率比较低！

2.使用case语句进行行列转换 select sum(case when user_name='孙悟空' then kills end) as '孙悟空', sum(case when user_name='猪八戒' then kills end) as '猪八戒', sum(case when user_name='沙僧' then kills end) as '沙僧' from user1 a join user_kills b on a.id=b.user_id

方法一 简单可以效率不高 修改也麻烦

1.判断数据是否重复： SELECT user_name , over, count(*) FROM user1_test GROUP BY user_name, over HAVING count(*)>1 2.删除重复数据，对于相同数据保留ID最大的 DELETE a FROM user1_test a JOIN( SELECT user_name,COUNT(*),MAX(id) AS id FROM user1_test GROUP BY user_name HAVING COUNT(*) > 1 ) b ON a.user_name = b.user_name WHERE a.id < b.id 3.将表中查询的数据再次插入到表中 INSERT INTO xmss_wh(XM_ID,WH_ID,YJ,SH,SHR,FJ_NAME,FJ_ID) SELECT XM_ID,WH_ID,YJ,SH,SHR,FJ_NAME,FJ_ID FROM xmss_wh WHERE ID=28; sql中的group by 用法解析： Group By语句从英文的字面意义上理解就是“根据(by)一定的规则进行分组(Group)”。 它的作用是通过一定的规则将一个数据集划分成若干个小的区域，然后针对若干个小区域进行数据处理。 --注意:group by 是先排序后分组； --举例子说明：如果要用到group by 一般用到的就是“每这个字” 例如说明现在有一个这样的表：每个部门有多少人 就要用到分组的技术 select DepartmentID as '部门名称', COUNT(*) as '个数' from BasicDepartment group by DepartmentID 这个就是使用了group by +字段进行了分组，其中我们就可以理解为我们按照了部门的名称ID DepartmentID将数据集进行了分组；然后再进行各个组的统计数据分别有多少； 如果不用count(*)等类似函数 select DepartmentID,DepartmentName from BasicDepartment group by DepartmentID 将会报错