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列轉(zhuǎn)行場(chǎng)景查看全部
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使用cross join 方式是實(shí)現(xiàn)行列轉(zhuǎn)換： 1.統(tǒng)計(jì)每一個(gè)用戶打怪的總數(shù) 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 '豬八戒' from user1 a left join user_kills b on a.id = b.user_id where a.username='豬八戒'; select sum(kills) as '孫悟空' from user1 a left join user_kills b on a.id = b.user_id where a.username='孫悟空'; 2.使用cross join 進(jìn)行笛卡爾積連接，將三組數(shù)據(jù)進(jìn)行行轉(zhuǎn)列組合 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 '豬八戒' from user1 a left join user_j=kills b on a.id = b.user_id and a.username='豬八戒') b cross join (select sum(kills) as '孫悟空' from user1 a left join user_j=kills b on a.id = b.user_id and a.username='孫悟空') c 總結(jié)：這種方法雖然可以轉(zhuǎn)換行列，但是遇到需要組合的數(shù)據(jù)太多編寫(xiě)起來(lái)就比較繁瑣，而且查詢效率比較低！查看全部
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2.使用case語(yǔ)句進(jìn)行行列轉(zhuǎn)換 select sum(case when user_name='孫悟空' then kills end) as '孫悟空', sum(case when user_name='豬八戒' then kills end) as '豬八戒', 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查看全部
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方法一 簡(jiǎn)單可以效率不高 修改也麻煩查看全部
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1.判斷數(shù)據(jù)是否重復(fù)： SELECT user_name , over, count(*) FROM user1_test GROUP BY user_name, over HAVING count(*)>1 2.刪除重復(fù)數(shù)據(jù)，對(duì)于相同數(shù)據(jù)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.將表中查詢的數(shù)據(jù)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語(yǔ)句從英文的字面意義上理解就是“根據(jù)(by)一定的規(guī)則進(jìn)行分組(Group)”。 它的作用是通過(guò)一定的規(guī)則將一個(gè)數(shù)據(jù)集劃分成若干個(gè)小的區(qū)域，然后針對(duì)若干個(gè)小區(qū)域進(jìn)行數(shù)據(jù)處理。 --注意:group by 是先排序后分組； --舉例子說(shuō)明：如果要用到group by 一般用到的就是“每這個(gè)字” 例如說(shuō)明現(xiàn)在有一個(gè)這樣的表：每個(gè)部門(mén)有多少人 就要用到分組的技術(shù) select DepartmentID as '部門(mén)名稱', COUNT(*) as '個(gè)數(shù)' from BasicDepartment group by DepartmentID 這個(gè)就是使用了group by +字段進(jìn)行了分組，其中我們就可以理解為我們按照了部門(mén)的名稱ID DepartmentID將數(shù)據(jù)集進(jìn)行了分組；然后再進(jìn)行各個(gè)組的統(tǒng)計(jì)數(shù)據(jù)分別有多少； 如果不用count(*)等類(lèi)似函數(shù) select DepartmentID,DepartmentName from BasicDepartment group by DepartmentID 將會(huì)報(bào)錯(cuò)查看全部
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列轉(zhuǎn)行查看全部
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sql行轉(zhuǎn)列的二查看全部
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行轉(zhuǎn)列的場(chǎng)景一報(bào)表統(tǒng)計(jì)查看全部
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sql技巧二查看全部
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刪除重復(fù)數(shù)據(jù) 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查看全部
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列轉(zhuǎn)行2 select user_name,'arms' as equipment, arms from user1 a join user1_equipment b on a.id=user_id union all select user_name,'clothing' as equipment, clothing from user1 a join user1_equipment b on a.id=user_id; union all select user_name,'shoe' as equipment, shoe from user1 a join user1_equipment b on a.id=user_id; ========================================== 列轉(zhuǎn)行3 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;查看全部
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列轉(zhuǎn)行技巧： 1.首先我們加一張序列號(hào)表，所謂序列號(hào)表就是一張有一個(gè)自增字段的表，如id 1,2,3,4,5,6...... 2.使用這條sql語(yǔ)句，這句有點(diǎn)整不懂。 SELECT user_name , REPLACE(SUBSTRING(SUBSTRING_INDEX(mobile,',',a.id),CHAR_LENGTH(SUBSTRING_INDEX(mobile,',',a.id-1))+1),',','') AS mobile FROM tb_sequence a CROSS JOIN ( SELECT user_name,CONCAT(mobile,','))+1 size FROM user1 b) b ON a.id <= b.size查看全部
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使用case語(yǔ)句進(jìn)行行列轉(zhuǎn)換 select sum(case when user_name='孫悟空' then kills end) as '孫悟空', sum(case when user_name='豬八戒' then kills end) as '豬八戒', 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查看全部
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刪除重復(fù)查看全部
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列轉(zhuǎn)行查看全部
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