SQL将行转换为列

用途:

  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,因为您将数据从行“透视”到列数据。

我实现了一个真正的动态函数来处理这个问题,而不必硬编码任何特定的答案类或使用外部模块/扩展。它还提供对列排序的完全控制,并支持多个键列和类/属性列。

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

解决这个特定问题的示例:

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;

此输出:

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

您可以使用 crosstab 以这种方式工作

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,必须安装TableFunc扩展。由于9.1版本,您可以使用创建扩展名执行此操作:

CREATE EXTENSION tablefunc;

我编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(如果它存在,则先删除它)。 您可以从视图中选择以获得结果。

功能如下:

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;

以下是我如何使用它:

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

例子: 拳头你跑:

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

比:

Select * from ct_view;

在中有一个这样的例子 contrib/tablefunc/ .