行列转换在做报表分析时还是经常会遇到的，今天就说一下如何实现行列转换吧。

行列转换就是如下图所示两种展示形式的互相转换

1. 行转列

我们来看一个简单的例子，我们要把下面这个表的数据，转换成图二的样式

image-20230914151818953.png

要转换的结果数据展示

image-20230914152642915.png

先看看建表语句：

CREATE TABLE tb_score_01( id INT(11) NOT NULL, userid VARCHAR(20) NOT NULL COMMENT '用户id', subject VARCHAR(20) COMMENT '科目', score DOUBLE COMMENT '成绩')DUPLICATE KEY(`id`)COMMENT 'OLAP'DISTRIBUTED BY HASH(`id`) BUCKETS 1PROPERTIES ("replication_allocation" = "tag.location.default: 1","in_memory" = "false","storage_format" = "V2","light_schema_change" = "true","disable_auto_compaction" = "false");INSERT INTO tb_score_01VALUES (1,'001','语文',90);INSERT INTO tb_score_01VALUES (2,'001','数学',92);INSERT INTO tb_score_01VALUES (3,'001','英语',80);INSERT INTO tb_score_01VALUES (4,'002','语文',88);INSERT INTO tb_score_01VALUES (5,'002','数学',90);INSERT INTO tb_score_01VALUES (6,'002','英语',75.5);INSERT INTO tb_score_01VALUES (7,'003','语文',70);INSERT INTO tb_score_01VALUES (8,'003','数学',85);INSERT INTO tb_score_01VALUES (9,'003','英语',90);INSERT INTO tb_score_01VALUES (10,'003','政治',82);

传统的做法我们大概是这样实现，一般是通过case when语句

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;或者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;

我们来看看 Doris 怎么实现这个行转列呢，有没有更简单、性能更好的一种方式

  1. 我们是不是可以首先将这个科目、成绩组成一个Map

  2. 然后在外层对这个 Map 进行遍历展开

  3. 从而完成这样一个行列转换呢

我们来看看实现

selectuserid, IFNULL(map['语文'],0) as '语文', IFNULL(map['英语'],0) as '英语', IFNULL(map['数学'],0) as '数学', IFNULL(map['政治'],0) as '政治'from( select userid ,map_agg(subject,score) as map from tb_score group by userid) t ;

这样实现上性能更好,我们来看一下效果

select -> userid, -> IFNULL(map['语文'],0) as '语文', -> IFNULL(map['英语'],0) as '英语', -> IFNULL(map['数学'],0) as '数学', -> IFNULL(map['政治'],0) as '政治' -> from( -> select userid ,map_agg(subject,score) as map from tb_score group by userid -> ) t ;+--------+--------+--------+--------+--------+| userid | 语文 | 英语 | 数学 | 政治 |+--------+--------+--------+--------+--------+| 001| 90 | 80 | 92 |0 || 002| 88 | 75.5 | 90 |0 || 003| 70 | 90 | 85 | 82 |+--------+--------+--------+--------+--------+3 rows in set (0.02 sec)

2. 列转行

实际使用中我们还有很多场景要把数据冲列转成行，下面我们来看一个例子，这个例子中每行是一个学生的，语文、数学、英语、政治的成绩，

image-20230914152642915.png

我们想转换成每门成绩都是独立的一行，转出的效果如下：

image-20230914152846996.png

我们来看看一个宽表转成高表我们之前的是怎么实现，一般我们是通过union all的方式，每科我们都是一个单独的SQL语句，然后将这些SQL Unoin all 在一起得到我们想要的结果。

SELECT userid,'语文' AS course,cn_score AS score FROM tb_score1UNION ALLSELECT userid,'数学' AS course,math_score AS score FROM tb_score1UNION ALLSELECT userid,'英语' AS course,en_score AS score FROM tb_score1UNION ALLSELECT userid,'政治' AS course,po_score AS score FROM tb_score1ORDER BY userid;

这样做的缺点：

  1. SQL 冗余

  2. 大量的union all 也会带来性能问题

我们来看看 Doris 怎么实现，首先 Doris 提供了 Lateral view，其实就是用来和像类似explode这种UDTF函数联用的，lateral view会将 UDTF生成的结果放到一个虚拟表中，然后这个虚拟表会和输入行进行join来达到连接UDTF 外的select字段的目的

还是以上面的例子来看，Doris我怎么对这个宽表转成高表，实现就是借助Lateral view

CREATE TABLE `tb_score1` ( `id` int(11) NOT NULL, `userid` varchar(20) NOT NULL COMMENT '用户id', `cn_score` double NULL COMMENT '语文成绩', `math_score` double NULL COMMENT '数学成绩', `en_score` double NULL COMMENT '英语成绩', `po_score` double NULL COMMENT '政治成绩') ENGINE=OLAPUNIQUE KEY(`id`)COMMENT 'OLAP'DISTRIBUTED BY HASH(`id`) BUCKETS 1PROPERTIES ("replication_allocation" = "tag.location.default: 1","is_being_synced" = "false","storage_format" = "V2","light_schema_change" = "true","disable_auto_compaction" = "false","enable_single_replica_compaction" = "false");;INSERT INTO `tb_score1` (`id`, `userid`, `cn_score`, `math_score`, `en_score`, `po_score`) VALUES (1, '001', 90, 92, 80, 0);INSERT INTO `tb_score1` (`id`, `userid`, `cn_score`, `math_score`, `en_score`, `po_score`) VALUES (2, '002', 88, 90, 75.5, 0);INSERT INTO `tb_score1` (`id`, `userid`, `cn_score`, `math_score`, `en_score`, `po_score`) VALUES (3, '003', 70, 85, 90, 82);
  1. 首先我借助Lateral view 形成一个 UserID、客户成绩组成一个字符（使用逗号连接）,达到下面的效果
+--------+--------------------+| userid | arr|+--------+--------------------+| 001| ["语文", "90"] || 001| ["数学", "92"] || 001| ["英语", "80"] || 001| ["政治", "0"]|| 002| ["语文", "88"] || 002| ["数学", "90"] || 002| ["英语", "75.5"] || 002| ["政治", "0"]|| 003| ["语文", "70"] || 003| ["数学", "85"] || 003| ["英语", "90"] || 003| ["政治", "82"] |+--------+--------------------+12 rows in set (0.02 sec)

  1. 然后对这个上面的 arr 字符串，借助于 Doris 提供的 SPLIT_BY_STRING 函数完成字符串转数组的动作

  2. 最后遍历数组

  3. 完成列转行的效果

SELECT userid, element_at ( arr, 1 ) AS SUBJECT, element_at ( arr, 2 ) AS score FROM ( SELECT userid, SPLIT_BY_STRING ( sub, ',' ) arrFROM ( SELECT userid, array ( concat( '语文', ',', cn_score ), concat( '数学', ',', math_score ), concat( '英语', ',', en_score ), concat( '政治', ',', po_score )) AS scoresFROM tb_score1) t LATERAL VIEW explode ( scores ) tbl1 AS sub) aaa

最后的效果如下：

SELECT -> userid, -> element_at ( arr, 1 ) AS SUBJECT, -> element_at ( arr, 2 ) AS score -> FROM -> ( -> SELECT -> userid, -> SPLIT_BY_STRING ( sub, ',' ) arr -> FROM -> ( -> SELECT -> userid, -> array ( -> concat( '语文', ',', cn_score ), -> concat( '数学', ',', math_score ), -> concat( '英语', ',', en_score ), -> concat( '政治', ',', po_score )) AS scores -> FROM -> tb_score1 -> ) t LATERAL VIEW explode ( scores ) tbl1 AS sub -> ) aaa;+--------+---------+-------+| userid | SUBJECT | score |+--------+---------+-------+| 001| 语文| 90|| 001| 数学| 92|| 001| 英语| 80|| 001| 政治| 0 || 002| 语文| 88|| 002| 数学| 90|| 002| 英语| 75.5|| 002| 政治| 0 || 003| 语文| 70|| 003| 数学| 85|| 003| 英语| 90|| 003| 政治| 82|+--------+---------+-------+12 rows in set (0.02 sec)

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