Apache Doris 行列转换可以这样玩

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 1
PROPERTIES (
"replication_allocation" = "tag.location.default: 1",
"in_memory" = "false",
"storage_format" = "V2",
"light_schema_change" = "true",
"disable_auto_compaction" = "false"
);
 
INSERT INTO tb_score_01  VALUES (1,'001','语文',90);
INSERT INTO tb_score_01  VALUES (2,'001','数学',92);
INSERT INTO tb_score_01  VALUES (3,'001','英语',80);
INSERT INTO tb_score_01  VALUES (4,'002','语文',88);
INSERT INTO tb_score_01  VALUES (5,'002','数学',90);
INSERT INTO tb_score_01  VALUES (6,'002','英语',75.5);
INSERT INTO tb_score_01  VALUES (7,'003','语文',70);
INSERT INTO tb_score_01  VALUES (8,'003','数学',85);
INSERT INTO tb_score_01  VALUES (9,'003','英语',90);
INSERT INTO tb_score_01  VALUES (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

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 ;

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_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=OLAP
UNIQUE KEY(`id`)
COMMENT 'OLAP'
DISTRIBUTED BY HASH(`id`) BUCKETS 1
PROPERTIES (
"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, ',' ) 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

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)

日记本