【Databend】行列转化：一行变多行和简单分列

数据准备和需求

行列转化在实际工作中很常见，其中最常见的有一行变多行，有下面一份数据：

drop table if exists fact_suject_data;
create table if not exists fact_suject_data
(
    student_id    int          null comment '编号',
    subject_level varchar null comment '科目等级',
    subject_level_json variant null comment '科目等级json数据'
);
insert into fact_suject_data(student_id, subject_level,subject_level_json) values (12,'china e,english d,math e','{"china": "e","english": "d","math": "e"}');
insert into fact_suject_data(student_id, subject_level,subject_level_json) values (2,'china b,english b','{"china": "b","english": "b"}');
insert into fact_suject_data(student_id, subject_level,subject_level_json) values (3,'english a,math c','{"english": "a","math": "c"}');
insert into fact_suject_data(student_id, subject_level,subject_level_json) values (4,'china c,math a','{"china": "c","math": "a"}');
insert into fact_suject_data(student_id, subject_level,subject_level_json) values (5,'china d,english a,math c','{"china": "d","english": "a","math": "c"}');
insert into fact_suject_data(student_id, subject_level,subject_level_json) values (6,'china c,english a,math d','{"china": "c","english": "a","math": "d"}');
insert into fact_suject_data(student_id, subject_level,subject_level_json) values (7,'china a,english e,math b','{"china": "a","english": "e","math": "b"}');
insert into fact_suject_data(student_id, subject_level,subject_level_json) values (8,'china d,english e,math e','{"china": "d","english": "e","math": "e"}');
insert into fact_suject_data(student_id, subject_level,subject_level_json) values (9,'china c,english e,math c','{"china": "c","english": "e","math": "c"}');

需求是将学生学科等级和等级分隔成多行，效果如下：

生成序列和分隔函数

Databend 生成序列有专门的函数 generate_series(, [, ])，生成从指定点开始，在另一个指定点结束的数据集，并且可以选择增量值。适用的数据类型有 整数、日期和时间戳。

select generate_series as n  from generate(1, 10);
select generate_series as n  from generate(1, 10, 2);
+---+
| n |
+---+
| 1 |
+---+
| 3 |
+---+
| 5 |
+---+
| 7 |
+---+
| 9 |
+---+
select generate_series as n from generate_series('2024-01-01'::date, '2024-01-07'::date);
+---------------+
| calendar_date |
+---------------+
|   2024-01-01  |
+---------------+
|   2024-01-02  |
+---------------+
|   2024-01-03  |
+---------------+
|   2024-01-04  |
+---------------+
|   2024-01-05  |
+---------------+
|   2024-01-06  |
+---------------+
|   2024-01-07  |
+---------------+

• split(,)：使用指定的分隔符拆分字符串，并将结果部分作为数组返回。
• split_part(,, )：使用指定的分隔符拆分字符串并返回指定的部分。
• unnest(array)：将数组拆分成多行。

select subject_level
     , split(subject_level, ',')         as split_char
     , split_part(subject_level, ',', 1) as part1
from (select 'china e,english d,math e' as subject_level) as a
+--------------------------+----------------------------------+------------+
|       subject_level      |             split_char           |   part1    | 
+--------------------------+----------------------------------+------------+
| china e,english d,math e | ['china e','english d','math e'] |  china e   |
+--------------------------+----------------------------------+------------+
select subject_level
     , unnest(split(subject_level, ',')) as unne_char
from (select 'china e,english d,math e' as subject_level) as a;
+--------------------------+------------+
|       subject_level      | unne_char  | 
+--------------------------+------------+
| china e,english d,math e |  china e   |
+--------------------------+------------+

split_part() 函数与 Mysql 中的 substring_index() 类似。

根据分隔符变多行

根据上面函数讲解，
方法一：我们可以使用 split(,) 和 unnest(array) 函数实现。

select t1.student_id,t1.subject_level
,unnest(split(t1.subject_level,',')) as subject_level1
from fact_suject_data as t1
order by t1.student_id;

方法二：也可以使用 split_part(,, ) 单独实现。

select t1.student_id
     , t1.subject_level
     , t2.n
     , split_part(t1.subject_level, ',', t2.n) as subject_level1
from fact_suject_data as t1
         left join (select generate_series as n from generate_series(1, 30)) t2
                   on t2.n <= (length(t1.subject_level) - length(replace(t1.subject_level, ',', '')) + 1)
order by t1.student_id;

通过 generate_series() 生成的序列数值作为 split_part() 的分隔参数即可实现，与 Mysql 行列变换《你想要的都有》中分隔原理一致。

JSON 数据简单分列

对于 subject_level_json 列数据，我们可以使用 json 独有的函数实现分列透视的效果。

select subject_level_json
     , replace(json_path_query(subject_level_json, '$.china'), '"', '') as china
     , get(subject_level_json, 'math')                                  as math
     , get(subject_level_json, 'english')                               as english
from fact_suject_data as t1
order by t1.student_id;

总结

数据分列和一行变多行的应用非常常见，通过本文的学习，相信基本上能处理类似问题，遇到面试相关问题也能完美解决，赶紧动手实操看看效果吧！！！

参考资料：

• Databend Array Functions：https://docs.databend.com/sql/sql-functions/array-functions/
• Mysql 行列变换《你想要的都有》：https://blog.csdn.net/weixin_50357986/article/details/134161183
• Databend 基础函数应用：https://blog.csdn.net/weixin_50357986/article/details/135535471