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更新时间：2024-06-21 GMT+08:00
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GaussDB(DWS)如何实现行转列及列转行？

本节介绍GaussDB(DWS)中如何使用SQL语句实现行转列、列转行。

场景介绍

以学生成绩为例：

老师会按照学科录入成绩，每科老师都会单独录入每个学生对应学科的成绩，而每位学生只关注自己各科的成绩。如果把老师录入数据作为原始表，那么学生查看自己的成绩就要用到行转列；如果让学生自己填写各科的成绩并汇总，然后老师去查自己学科所有学生的成绩，那就是列转行。

行转列与列转行的示意图如下：

图1 示意图
  • 行转列

    将多行数据转换成一行显示，或将一列数据转换成多列显示。

  • 列转行

    将一行数据转换成多行显示，或将多列数据转换成一列显示。

示例表

  • 创建行存表students_info并插入数据。 
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    CREATE TABLE students_info(name varchar(20),subject varchar(100),score bigint) distribute by hash(name);
INSERT INTO students_info VALUES('lily','math',95);
INSERT INTO students_info VALUES('lily','physics',80);
INSERT INTO students_info VALUES('lily','literature',92);
INSERT INTO students_info VALUES('matu','math',75);
INSERT INTO students_info VALUES('matu','physics',90);
INSERT INTO students_info VALUES('matu','literature',85);
INSERT INTO students_info VALUES('jack','math',90);
INSERT INTO students_info VALUES('jack','physics',95);
INSERT INTO students_info VALUES('jack','literature',95);
    查看表students_info信息。 
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    SELECT * FROM students_info;
 name |  subject   | score
------+------------+-------
 matu | math       |    75
 matu | physics    |    90
 matu | literature |    85
 lily | math       |    95
 lily | physics    |    80
 lily | literature |    92
 jack | math       |    90
 jack | physics    |    95
 jack | literature |    95
  • 创建列存表students_info1并插入数据。 
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    CREATE TABLE students_info1(name varchar(20), math bigint, physics bigint, literature bigint) with (orientation = column) distribute by hash(name);
INSERT INTO students_info1 VALUES('lily',95,80,92);
INSERT INTO students_info1 VALUES('matu',75,90,85);
INSERT INTO students_info1 VALUES('jack',90,95,95);

    查看表students_info1信息。

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    SELECT * FROM students_info1;
 name | math | physics | literature
------+------+---------+------------
 matu |   75 |      90 |         85
 lily |   95 |      80 |         92
 jack |   90 |      95 |         95
(3 rows)

静态行转列

静态行转列需要手动指定每一列的列名，如果存在则取其对应值，否则将赋其默认值0。

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SELECT name,
sum(case when subject='math' then score else 0 end) as math,
sum(case when subject='physics' then score else 0 end) as physics,
sum(case when subject='literature' then score else 0 end) as literature FROM students_info GROUP BY name;
 name | math | physics | literature
------+------+---------+------------
 matu |   75 |      90 |         85
 lily |   95 |      80 |         92
 jack |   90 |      95 |         95
(3 rows)

动态行转列

8.1.2及以上集群版本可使用GROUP_CONCAT生成列存语句。

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SELECT group_concat(concat('sum(IF(subject = ''', subject, ''', score, 0)) AS "', name, '"'))FROM students_info;
      group_concat

-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
------------------------
 sum(IF(subject = 'literature', score, 0)) AS "jack",sum(IF(subject = 'literature', score, 0)) AS "lily",sum(IF(subject = 'literature', score, 0)) AS "matu",sum(IF(subject = 'math', score, 0)) AS "jack",sum(IF
(subject = 'math', score, 0)) AS "lily",sum(IF(subject = 'math', score, 0)) AS "matu",sum(IF(subject = 'physics', score, 0)) AS "jack",sum(IF(subject = 'physics', score, 0)) AS "lily",sum(IF(subject = 'physics
', score, 0)) AS "matu"
(1 row)

8.1.1及更低版本中可用LISTAGG生成列存语句。

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SELECT listagg(concat('sum(case when subject = ''', subject, ''' then score else 0 end) AS "', subject, '"'),',') within GROUP(ORDER BY 1)FROM (select distinct subject from students_info);
                                                                                                      listagg

-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
--
 sum(case when subject = 'literature' then score else 0 end) AS "literature",sum(case when subject = 'physics' then score else 0 end) AS "physics",sum(case when subject = 'math' then score else 0 end) AS "math
"
(1 row)

再通过视图动态重建：

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CREATE OR REPLACE FUNCTION build_view()
RETURNS VOID
LANGUAGE plpgsql
AS $$ DECLARE
sql text;
rec record;
BEGIN
sql := 'select LISTAGG(
	CONCAT( ''sum(case when subject = '''''', subject, '''''' then score else 0 end) AS "'', subject, ''"'' )
	,'','' ) within group(order by 1) from (select distinct subject from students_info);';
	EXECUTE sql INTO rec;
	sql := 'drop view if exists get_score';
	EXECUTE sql;
	sql := 'create view get_score as select name, ' || rec.LISTAGG || ' from students_info group by name';
	EXECUTE sql;
END$$;

执行重建：

1
 
CALL build_view();

查询视图：

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SELECT * FROM get_score;
 name | literature | physics | math
------+------------+---------+------
 matu |         85 |      90 |   75
 lily |         92 |      80 |   95
 jack |         95 |      95 |   90
(3 rows)

列转行

使用union all，将各科目（math、physics和literature）整合为一列，示例如下：

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SELECT * FROM 
(
SELECT name, 'math' AS subject, math AS score FROM students_info1
union all
SELECT name, 'physics' AS subject, physics AS score FROM students_info1
union all
SELECT name, 'literature' AS subject, literature AS score FROM students_info1
) 
order by name;
 name |  subject   | score
------+------------+-------
 jack | math       |    90
 jack | physics    |    95
 jack | literature |    95
 lily | math       |    95
 lily | physics    |    80
 lily | literature |    92
 matu | math       |    75
 matu | physics    |    90
 matu | literature |    85
(9 rows)
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