例如:把图1转换成图2结果展示
图1如下:
图2如下:
建表语句:
DROP TABLE IF EXISTS tb_score; CREATE TABLE tb_score( id INT(11) NOT NULL auto_increment, userid VARCHAR(20) NOT NULL COMMENT '用户id', subject VARCHAR(20) COMMENT '科目', score DOUBLE COMMENT '成绩', PRIMARY KEY(id) )ENGINE = INNODB DEFAULT CHARSET = utf8;
插入数据:
INSERT INTO tb_score(userid,subject,score) VALUES ('001','语文',90); INSERT INTO tb_score(userid,subject,score) VALUES ('001','数学',92); INSERT INTO tb_score(userid,subject,score) VALUES ('001','英语',80); INSERT INTO tb_score(userid,subject,score) VALUES ('002','语文',88); INSERT INTO tb_score(userid,subject,score) VALUES ('002','数学',90); INSERT INTO tb_score(userid,subject,score) VALUES ('002','英语',75.5); INSERT INTO tb_score(userid,subject,score) VALUES ('003','语文',70); INSERT INTO tb_score(userid,subject,score) VALUES ('003','数学',85); INSERT INTO tb_score(userid,subject,score) VALUES ('003','英语',90); INSERT INTO tb_score(userid,subject,score) VALUES ('003','政治',82);
1、使用case...when....then 进行行转列
SELECT userid, SUM(CASE `subject` WHEN '语文' THEN score ELSE 0 END) as '语文', SUM(CASE `subject` WHEN '数学' THEN score ELSE 0 END) as '数学', SUM(CASE `subject` WHEN '英语' THEN score ELSE 0 END) as '英语', SUM(CASE `subject` WHEN '政治' THEN score ELSE 0 END) as '政治' FROM tb_score GROUP BY userid
2、使用IF() 进行行转列:
SELECT userid, SUM(IF(`subject`='语文',score,0)) as '语文', SUM(IF(`subject`='数学',score,0)) as '数学', SUM(IF(`subject`='英语',score,0)) as '英语', SUM(IF(`subject`='政治',score,0)) as '政治' FROM tb_score GROUP BY userid
注意点:
(1)sum() 是为了能够使用GROUP BY根据userid进行分组,因为每一个userid对应的subject="语文"的记录只有一条,所以SUM() 的值就等于对应那一条记录的score的值。
假如userid ='001' and subject='语文' 的记录有两条,则此时SUM() 的值将会是这两条记录的和,同理,使用Max()的值将会是这两条记录里面值最大的一个。但是正常情况下,一个user对应一个subject只有一个分数,因此可以使用SUM()、MAX()、MIN()、AVG()等聚合函数都可以达到行转列的效果。
(2)IF(subject='语文',score,0) 作为条件,即对所有subject='语文'的记录的score字段进行SUM()、MAX()、MIN()、AVG()操作,如果score没有值则默认为0。
上面图2转成图1
建表语句:
CREATE TABLE tb_score1( id INT(11) NOT NULL auto_increment, userid VARCHAR(20) NOT NULL COMMENT '用户id', cn_score DOUBLE COMMENT '语文成绩', math_score DOUBLE COMMENT '数学成绩', en_score DOUBLE COMMENT '英语成绩', po_score DOUBLE COMMENT '政治成绩', PRIMARY KEY(id) )ENGINE = INNODB DEFAULT CHARSET = utf8;
插入数据:
INSERT INTO tb_score1(userid,cn_score,math_score,en_score,po_score) VALUES ('001',90,92,80,0); INSERT INTO tb_score1(userid,cn_score,math_score,en_score,po_score) VALUES ('002',88,90,75.5,0); INSERT INTO tb_score1(userid,cn_score,math_score,en_score,po_score) VALUES ('003',70,85,90,82);
本质是将userid的每个科目分数分散成一条记录显示出来。
SELECT userid,'语文' AS course,cn_score AS score FROM tb_score1 UNION ALL SELECT userid,'数学' AS course,math_score AS score FROM tb_score1 UNION ALL SELECT userid,'英语' AS course,en_score AS score FROM tb_score1 UNION ALL SELECT userid,'政治' AS course,po_score AS score FROM tb_score1 ORDER BY userid
这里将每个userid对应的多个科目的成绩查出来,通过UNION ALL将结果集加起来,达到上图的效果。
附:UNION与UNION ALL的区别(摘):
1.对重复结果的处理:UNION会去掉重复记录,UNION ALL不会;
2.对排序的处理:UNION会排序,UNION ALL只是简单地将两个结果集合并;
3.效率方面的区别:因为UNION 会做去重和排序处理,因此效率比UNION ALL慢很多;