原文: http://blog.csdn.net/jx_870915876/article/details/52403472
add by zhj: 本文是以MySQL为例说明的,但其实它适用于所有关系型数据库。
行转列的需求如下,可以发现,行转列时其实是先group by,然后将每个组中的行转为列,使用case-when + 聚合函数,
每个由行转为列的列,都要使用case-when或when-case语句。case-when是怎么作用的呢?组中的每行都经过case-when,
如果组中有N行,那通过case-when就产生N个值,再使用聚合函数,变为一个单一值,即新列的值。
行转列
例如:把图1转换成图2结果展示
图1
图2
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CREATE TABLE `TEST_TB_GRADE` (
`ID` int (10) NOT NULL AUTO_INCREMENT,
`USER_NAME` varchar (20) DEFAULT NULL ,
`COURSE` varchar (20) DEFAULT NULL ,
`SCORE` float DEFAULT '0' ,
PRIMARY KEY (`ID`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
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insert into TEST_TB_GRADE(USER_NAME, COURSE, SCORE) values
( "张三" , "数学" , 34),
( "张三" , "语文" , 58),
( "张三" , "英语" , 58),
( "李四" , "数学" , 45),
( "李四" , "语文" , 87),
( "李四" , "英语" , 45),
( "王五" , "数学" , 76),
( "王五" , "语文" , 34),
( "王五" , "英语" , 89);
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行转列SQL:
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SELECT user_name ,
MAX ( CASE course WHEN '数学' THEN score ELSE 0 END ) 数学,
MAX ( CASE course WHEN '语文' THEN score ELSE 0 END ) 语文,
MAX ( CASE course WHEN '英语' THEN score ELSE 0 END ) 英语
FROM test_tb_grade
GROUP BY USER_NAME;
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列转行
例如:把图1转换成图3结果展示
图3
列转行SQL:
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select user_name, '语文' COURSE , CN_SCORE as SCORE from test_tb_grade2
union select user_name, '数学' COURSE, MATH_SCORE as SCORE from test_tb_grade2
union select user_name, '英语' COURSE, EN_SCORE as SCORE from test_tb_grade2
order by user_name,COURSE;
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