sql - SQL 将行转置为列

Question

我有一个有趣的难题，我相信可以用纯 SQL 来解决。我有类似于以下的表格：

responses:

user_id | question_id | body
----------------------------
1       | 1           | Yes
2       | 1           | Yes
1       | 2           | Yes
2       | 2           | No
1       | 3           | No
2       | 3           | No


questions:

id | body
-------------------------
1 | Do you like apples?
2 | Do you like oranges?
3 | Do you like carrots?

我想得到以下输出

user_id | Do you like apples? | Do you like oranges? | Do you like carrots?
---------------------------------------------------------------------------
1       | Yes                 | Yes                  | No
2       | Yes                 | No                   | No

我不知道会有多少问题，而且它们是动态的，所以我不能只为每个问题编写代码。我正在使用 PostgreSQL，我相信这称为转置，但我似乎找不到任何说明在 SQL 中执行此操作的标准方法的任何内容。我记得在大学的数据库课上这样做过，但那是在 MySQL 中，老实说，我不记得我们是怎么做到的。

我假设它将是连接和GROUP BY语句的组合，但我什至不知道如何开始。

有人知道怎么做吗？非常感谢！

编辑1：我发现了一些关于使用交叉表的信息，这似乎是我想要的，但我无法理解它。链接到更好的文章将不胜感激！

Answer 1

利用：

  SELECT r.user_id,
         MAX(CASE WHEN r.question_id = 1 THEN r.body ELSE NULL END) AS "Do you like apples?",
         MAX(CASE WHEN r.question_id = 2 THEN r.body ELSE NULL END) AS "Do you like oranges?",
         MAX(CASE WHEN r.question_id = 3 THEN r.body ELSE NULL END) AS "Do you like carrots?"
    FROM RESPONSES r
    JOIN QUESTIONS q ON q.id = r.question_id
GROUP BY r.user_id

这是一个标准的透视查询，因为您正在将数据从行“透视”到列数据。

Answer 2

我实现了一个真正动态的函数来处理这个问题，而无需硬编码任何特定类别的答案或使用外部模块/扩展。它还可以完全控制列排序并支持多个键和类/属性列。

你可以在这里找到它：https ://github.com/jumpstarter-io/colpivot

解决此特定问题的示例：

begin;

create temporary table responses (
    user_id integer,
    question_id integer,
    body text
) on commit drop;

create temporary table questions (
    id integer,
    body text
) on commit drop;

insert into responses values (1,1,'Yes'), (2,1,'Yes'), (1,2,'Yes'), (2,2,'No'), (1,3,'No'), (2,3,'No');
insert into questions values (1, 'Do you like apples?'), (2, 'Do you like oranges?'), (3, 'Do you like carrots?');

select colpivot('_output', $$
    select r.user_id, q.body q, r.body a from responses r
        join questions q on q.id = r.question_id
$$, array['user_id'], array['q'], '#.a', null);

select * from _output;

rollback;

这输出：

 user_id | 'Do you like apples?' | 'Do you like carrots?' | 'Do you like oranges?' 
---------+-----------------------+------------------------+------------------------
       1 | Yes                   | No                     | Yes
       2 | Yes                   | No                     | No

Answer 3

您可以通过这种方式使用交叉表函数解决此示例

drop table if exists responses;
create table responses (
user_id integer,
question_id integer,
body text
);

drop table if exists questions;
create table questions (
id integer,
body text
);

insert into responses values (1,1,'Yes'), (2,1,'Yes'), (1,2,'Yes'), (2,2,'No'), (1,3,'No'), (2,3,'No');
insert into questions values (1, 'Do you like apples?'), (2, 'Do you like oranges?'), (3, 'Do you like carrots?');

select * from crosstab('select responses.user_id, questions.body, responses.body from responses, questions where questions.id = responses.question_id order by user_id') as ct(userid integer, "Do you like apples?" text, "Do you like oranges?" text, "Do you like carrots?" text);

首先，您必须安装 tablefunc 扩展。从 9.1 版本开始，您可以使用 create extension 来实现：

CREATE EXTENSION tablefunc;

Answer 4

我写了一个函数来生成动态查询。它为交叉表生成 sql 并创建一个视图（如果存在则首先删除它）。您可以从视图中进行选择以获取结果。

这是功能：

CREATE OR REPLACE FUNCTION public.c_crosstab (
  eavsql_inarg varchar,
  resview varchar,
  rowid varchar,
  colid varchar,
  val varchar,
  agr varchar
)
RETURNS void AS
$body$
DECLARE
    casesql varchar;
    dynsql varchar;    
    r record;
BEGIN   
 dynsql='';

 for r in 
      select * from pg_views where lower(viewname) = lower(resview)
  loop
      execute 'DROP VIEW ' || resview;
  end loop;   

 casesql='SELECT DISTINCT ' || colid || ' AS v from (' || eavsql_inarg || ') eav ORDER BY ' || colid;
 FOR r IN EXECUTE casesql Loop
    dynsql = dynsql || ', ' || agr || '(CASE WHEN ' || colid || '=''' || r.v || ''' THEN ' || val || ' ELSE NULL END) AS ' || agr || '_' || r.v;
 END LOOP;
 dynsql = 'CREATE VIEW ' || resview || ' AS SELECT ' || rowid || dynsql || ' from (' || eavsql_inarg || ') eav GROUP BY ' || rowid;
 RAISE NOTICE 'dynsql %1', dynsql; 
 EXECUTE dynsql;
END

$body$
LANGUAGE 'plpgsql'
VOLATILE
CALLED ON NULL INPUT
SECURITY INVOKER
COST 100;

这是我使用它的方式：

SELECT c_crosstab('query_txt', 'view_name', 'entity_column_name', 'attribute_column_name', 'value_column_name', 'first');

例子：你跑的拳头：

SELECT c_crosstab('Select * from table', 'ct_view', 'usr_id', 'question_id', 'response_value', 'first');

比：

Select * from ct_view;

Answer 5

中有一个例子contrib/tablefunc/。