一、行转列
1、建表
CREATE TABLE score( student_id VARCHAR(20) NOT NULL COMMENT ‘学生编号‘ DEFAULT ‘‘, student_name VARCHAR(50) NOT NULL COMMENT ‘学生姓名‘ DEFAULT ‘‘, gender VARCHAR(10) NOT NULL COMMENT ‘学生性别‘ DEFAULT ‘‘, subject_name VARCHAR(50) NOT NULL COMMENT ‘课程名称‘ DEFAULT ‘‘, score INTEGER NOT NULL COMMENT ‘分数‘ DEFAULT 0 )ENGINE=MyISAM DEFAULT CHARSET=gbk COMMENT=‘学生成绩‘; DELETE FROM score; INSERT INTO score VALUES(‘S001‘,‘张三‘,‘男‘,‘高等数学‘,82); INSERT INTO score VALUES(‘S001‘,‘张三‘,‘男‘,‘计算机导论‘,67); INSERT INTO score VALUES(‘S001‘,‘张三‘,‘男‘,‘概率论‘,90); INSERT INTO score VALUES(‘S001‘,‘张三‘,‘男‘,‘机械原理‘,82); INSERT INTO score VALUES(‘S002‘,‘李四‘,‘男‘,‘高等数学‘,78); INSERT INTO score VALUES(‘S002‘,‘李四‘,‘男‘,‘计算机导论‘,76); INSERT INTO score VALUES(‘S002‘,‘李四‘,‘男‘,‘概率论‘,65); INSERT INTO score VALUES(‘S002‘,‘李四‘,‘男‘,‘几何学‘,43); INSERT INTO score VALUES(‘S003‘,‘王五‘,‘女‘,‘计算机导论‘,88); INSERT INTO score VALUES(‘S003‘,‘王五‘,‘女‘,‘概率论‘,98); INSERT INTO score VALUES(‘S003‘,‘王五‘,‘女‘,‘几何学‘,85); INSERT INTO score VALUES(‘S004‘,‘赵六‘,‘男‘,‘高等数学‘,84); INSERT INTO score VALUES(‘S004‘,‘赵六‘,‘男‘,‘计算机导论‘,76); INSERT INTO score VALUES(‘S004‘,‘赵六‘,‘男‘,‘机械原理‘,65); INSERT INTO score VALUES(‘S004‘,‘赵六‘,‘男‘,‘几何学‘,48); INSERT INTO score VALUES(‘S005‘,‘孙七‘,‘女‘,‘高等数学‘,34); INSERT INTO score VALUES(‘S005‘,‘孙七‘,‘女‘,‘计算机导论‘,91); INSERT INTO score VALUES(‘S005‘,‘孙七‘,‘女‘,‘概率论‘,82); INSERT INTO score VALUES(‘S005‘,‘孙七‘,‘女‘,‘机械原理‘,56); INSERT INTO score VALUES(‘S005‘,‘孙七‘,‘女‘,‘几何学‘,70);
2、利用max(CASE ... WHEN ... THEN .. ELSE END) AS ""的方式来实现
##利用max(CASE ... WHEN ... THEN .. ELSE END) AS 的方式来实现 ##判断如果是这门学科，就取它的成绩，否则赋值为0，然后在成绩与0里取最大值 SELECT student_id, student_name, MAX(CASE WHEN subject_name = ‘高等数学‘ THEN score ELSE 0 END) AS ‘高等数学‘, MAX(CASE WHEN subject_name = ‘计算机导论‘ THEN score ELSE 0 END) AS ‘计算机导论‘, MAX(CASE WHEN subject_name = ‘概率论‘ THEN score ELSE 0 END) AS ‘概率论‘, MAX(CASE WHEN subject_name = ‘机械原理‘ THEN score ELSE 0 END) AS ‘机械原理‘, MAX(CASE WHEN subject_name = ‘几何学‘ THEN score ELSE 0 END) AS ‘几何学‘ FROM score GROUP BY student_id, student_name ORDER BY student_id, student_name;
3、求男女生各科平均成绩
##平均成绩肯定是总分除以人数，但是呢，有的学生没有某一门学科的成绩，我们把它变成0， ##变成0其实是不妥当的，因为变成0，在做除法的时候，也会算成一个人，这样除的结果就会不准确 ##所以应该把0变成null SELECT gender, AVG(CASE WHEN subject_name = ‘高等数学‘ THEN score ELSE NULL END) AS ‘高等数学‘, AVG(CASE WHEN subject_name = ‘计算机导论‘ THEN score ELSE NULL END) AS ‘计算机导论‘, AVG(CASE WHEN subject_name = ‘概率论‘ THEN score ELSE NULL END) AS ‘概率论‘, AVG(CASE WHEN subject_name = ‘机械原理‘ THEN score ELSE NULL END) AS ‘机械原理‘, AVG(CASE WHEN subject_name = ‘几何学‘ THEN score ELSE NULL END) AS ‘几何学‘ FROM score GROUP BY gender ORDER BY gender; 总结： 行转列，分组(GROUP BY)的列必须是除需要行转列之外的业务主键。 例如tb表中业务主键应该是cname和cource,但是cource需要进行行转列，所以需要按照cname分组。
4、子查询的方式，求男女生各科成绩平均值
## SELECT gender, MAX(CASE WHEN subject_name = ‘高等数学‘ THEN score_avg ELSE 0 END) AS ‘高等数学‘, MAX(CASE WHEN subject_name = ‘计算机导论‘ THEN score_avg ELSE 0 END) AS ‘计算机导论‘, MAX(CASE WHEN subject_name = ‘概率论‘ THEN score_avg ELSE 0 END) AS ‘概率论‘, MAX(CASE WHEN subject_name = ‘机械原理‘ THEN score_avg ELSE 0 END) AS ‘机械原理‘, MAX(CASE WHEN subject_name = ‘几何学‘ THEN score_avg ELSE 0 END) AS ‘几何学‘ FROM ( SELECT gender, subject_name, AVG(score) AS score_avg FROM score GROUP BY gender,subject_name )a GROUP BY gender ORDER BY gender;
5、GROUP_CONCAT
GROUP_CONCAT()：该函数返回带有来自一个分组的连接的非NULL值的字符串结果。 SELECT student_id, student_name, GROUP_CONCAT(subject_name) AS 课程名称, GROUP_CONCAT(score) AS 分数 FROM score GROUP BY student_id, student_name;
结果：
二、列转行
1、建表
### employee_sales表： CREATE TABLE employee_sales( employee_id VARCHAR(20) NOT NULL COMMENT ‘员工编号‘ DEFAULT ‘‘, employee_name VARCHAR(50) NOT NULL COMMENT ‘员工姓名‘ DEFAULT ‘‘, sales_amount_q1 DECIMAL(18,2) NOT NULL COMMENT ‘一季度销售金额‘ DEFAULT 0, sales_amount_q2 DECIMAL(18,2) NOT NULL COMMENT ‘二季度销售金额‘ DEFAULT 0, sales_amount_q3 DECIMAL(18,2) NOT NULL COMMENT ‘三季度销售金额‘ DEFAULT 0, sales_amount_q4 DECIMAL(18,2) NOT NULL COMMENT ‘四季度销售金额‘ DEFAULT 0 )ENGINE=MyISAM DEFAULT CHARSET=gbk COMMENT=‘员工销售表‘; INSERT INTO employee_sales VALUES(‘S001‘,‘张三‘,273193.41,84149.96,711798.18,847930.56); INSERT INTO employee_sales VALUES(‘S002‘,‘李四‘,861586.57,819398.89,496788.47,1060.11); INSERT INTO employee_sales VALUES(‘S003‘,‘王五‘,943899.97,793446.45,351139,640459.72); INSERT INTO employee_sales VALUES(‘S004‘,‘赵六‘,585158.08,493034.64,576246.61,188906.14); ### employee_assessment表： CREATE TABLE employee_assessment( employee_id VARCHAR(20) NOT NULL COMMENT ‘员工编号‘ DEFAULT ‘‘, employee_name VARCHAR(50) NOT NULL COMMENT ‘员工姓名‘ DEFAULT ‘‘, assessment_level CHAR(4) NOT NULL COMMENT ‘季度考核等级‘ DEFAULT ‘‘, sales_amount VARCHAR(500) NOT NULL COMMENT ‘销售金额‘ DEFAULT ‘‘ )ENGINE=MyISAM DEFAULT CHARSET=gbk COMMENT=‘员工考核表‘; INSERT INTO employee_assessment VALUES(‘S001‘,‘张三‘,‘AABC‘,‘273193.41,84149.96,711798.18,847930.56‘); INSERT INTO employee_assessment VALUES(‘S002‘,‘李四‘,‘CBAB‘,‘861586.57,819398.89,496788.47,1060.11‘); INSERT INTO employee_assessment VALUES(‘S003‘,‘王五‘,‘BBAC‘,‘943899.97,793446.45,351139,640459.72‘); INSERT INTO employee_assessment VALUES(‘S004‘,‘赵六‘,‘BACA‘,‘585158.08,493034.64,576246.61,188906.14‘);
2、UNION ALL 列转行
##union all 合并多个子集 ##加了一个季度字段，方便查看 select employee_id,employee_name,‘一季度‘ as 季度,sales_amount_q1 from employee_sales union all select employee_id,employee_name,‘二季度‘ as 季度,sales_amount_q2 from employee_sales UNION ALL SELECT employee_id,employee_name,‘三季度‘ as 季度,sales_amount_q3 from employee_sales UNION ALL select employee_id,employee_name,‘四季度‘ as 季度,sales_amount_q4 from employee_sales ORDER BY employee_id;
3、列转行2
## 先看employee_assessment表数据： +-------------+---------------+------------------+-----------------------------------------+ | employee_id | employee_name | assessment_level | sales_amount | +-------------+---------------+------------------+-----------------------------------------+ | S001 | 张三 | AABC | 273193.41,84149.96,711798.18,847930.56 | | S002 | 李四 | CBAB | 861586.57,819398.89,496788.47,1060.11 | | S003 | 王五 | BBAC | 943899.97,793446.45,351139,640459.72 | | S004 | 赵六 | BACA | 585158.08,493034.64,576246.61,188906.14 | +-------------+---------------+------------------+-----------------------------------------+ ## 用union all列转行： select employee_id,employee_name,assessment_level,‘1‘ as 季度,right(left(assessment_level,1),1) from employee_assessment union all select employee_id,employee_name,assessment_level,‘2‘ as 季度,right(left(assessment_level,2),1) from employee_assessment union all select employee_id,employee_name,assessment_level,‘3‘ as 季度,right(left(assessment_level,3),1) from employee_assessment union all select employee_id,employee_name,assessment_level,‘4‘ as 季度,right(left(assessment_level,4),1) from employee_assessment; 结果： +-------------+---------------+------------------+--------+-----------------------------------+ | employee_id | employee_name | assessment_level | 季度 | right(left(assessment_level,1),1) | +-------------+---------------+------------------+--------+-----------------------------------+ | S001 | 张三 | AABC | 1 | A | | S002 | 李四 | CBAB | 1 | C | | S003 | 王五 | BBAC | 1 | B | | S004 | 赵六 | BACA | 1 | B | | S001 | 张三 | AABC | 2 | A | | S002 | 李四 | CBAB | 2 | B | | S003 | 王五 | BBAC | 2 | B | | S004 | 赵六 | BACA | 2 | A | | S001 | 张三 | AABC | 3 | B | | S002 | 李四 | CBAB | 3 | A | | S003 | 王五 | BBAC | 3 | A | | S004 | 赵六 | BACA | 3 | C | | S001 | 张三 | AABC | 4 | C | | S002 | 李四 | CBAB | 4 | B | | S003 | 王五 | BBAC | 4 | C | | S004 | 赵六 | BACA | 4 | A | +-------------+---------------+------------------+--------+-----------------------------------+ 上面用union all虽然可以得到结果，但是当数据多的时候，就要写很多行union all，不太好？
可以用两个表的笛卡尔积：
## 建sequence 表： 此表中的数据要和要拆分的字段数相同： CREATE TABLE sequence (seq_num INT); INSERT INTO sequence VALUES (1), (2), (3), (4); ## right和left函数组合使用，获取某一位字符：
加where条件是因为，现在assessment_level字段正好和sequence表中的字段数相等，但是当两个字段数不等时，比如assessment_level只有三个，加上where就可以起过滤作用，如：length(assessment_level) = 3,3已经不>=seq_num了，就算seq_num中有4，也不会select出来； select employee_id,employee_name,assessment_level,seq_num as 季度,right(left(assessment_level,seq_num),1) from employee_assessment,sequence where length(assessment_level) >= seq_num; 结果： +-------------+---------------+------------------+--------+-----------------------------------------+ | employee_id | employee_name | assessment_level | 季度 | right(left(assessment_level,seq_num),1) | +-------------+---------------+------------------+--------+-----------------------------------------+ | S001 | 张三 | AABC | 1 | A | | S001 | 张三 | AABC | 2 | A | | S001 | 张三 | AABC | 3 | B | | S001 | 张三 | AABC | 4 | C | | S002 | 李四 | CBAB | 1 | C | | S002 | 李四 | CBAB | 2 | B | | S002 | 李四 | CBAB | 3 | A | | S002 | 李四 | CBAB | 4 | B | | S003 | 王五 | BBAC | 1 | B | | S003 | 王五 | BBAC | 2 | B | | S003 | 王五 | BBAC | 3 | A | | S003 | 王五 | BBAC | 4 | C | | S004 | 赵六 | BACA | 1 | B | | S004 | 赵六 | BACA | 2 | A | | S004 | 赵六 | BACA | 3 | C | | S004 | 赵六 | BACA | 4 | A | +-------------+---------------+------------------+--------+-----------------------------------------+ 上面已经很简便的得到结果了，其实mysql还有一个函数：substring函数，能实现right和left相同的功能； ##substring函数，结果应该和上面一样 select employee_id,employee_name,assessment_level,seq_num as 季度,substring(assessment_level,seq_num,1) from employee_assessment,sequence where length(assessment_level) >= seq_num;
上面其实只是处理了assessment_level字段，sales_amount 字段还没有处理，值是以逗号分隔的 ：
##mysql有这样一个函数：substring_index函数 ##substring_index有三个参数：第一个是：要分隔的字段，第二个是：以什么分隔，第三个是：要取第几个逗号前面的数据 ##还嵌套了一层substring_index，-1：从右边往前取一个，以逗号分隔 ##这里的where，是先计算出逗号的个数，然后加1，在去和sequence表中的数据比较 select employee_id,employee_name,assessment_level,seq_num,sales_amount,substring_index(substring_index(sales_amount,‘,‘,seq_num),‘,‘,-1) from employee_assessment,sequence WHERE LENGTH(sales_amount) - LENGTH(REPLACE(sales_amount,‘,‘,‘‘))+1 >= seq_num ORDER BY employee_id;
原文：https://www.cnblogs.com/weiyiming007/p/11458521.html