[英]SQL Transpose Rows as Columns
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
我不知道会有多少个问题,它们是动态的,所以我不能只为每个问题编写代码。 我使用的是PostgreSQL,我相信这称为转置,但是我似乎找不到任何能说明在SQL中执行此操作的标准方法的内容。 我记得我上大学时在数据库课上这样做,但是那是在MySQL中做的,老实说我不记得我们是如何做到的。
我假设它将是联接和GROUP BY
语句的组合,但是我什至不知道如何开始。
有人知道该怎么做吗? 非常感谢!
编辑1:我发现了一些有关使用交叉表的信息,这似乎是我想要的,但是我很难理解它。 链接到更好的文章将不胜感激!
采用:
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
这是标准的透视查询,因为您是将数据从行“透视”到列数据。
我实现了一个真正的动态函数来处理此问题,而无需对任何特定类别的答案进行硬编码或使用外部模块/扩展。 它还可以完全控制列的排序,并支持多个键和类/属性列。
您可以在这里找到它: 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;
输出:
user_id | 'Do you like apples?' | 'Do you like carrots?' | 'Do you like oranges?'
---------+-----------------------+------------------------+------------------------
1 | Yes | No | Yes
2 | Yes | No | No
您可以通过交叉表函数以这种方式解决此示例
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;
我编写了一个函数来生成动态查询。 它为交叉表生成sql并创建视图(如果存在,则将其首先删除)。 然后,您可以从视图中选择以获取结果。
这是函数:
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;
这是我的用法:
SELECT c_crosstab('query_txt', 'view_name', 'entity_column_name', 'attribute_column_name', 'value_column_name', 'first');
示例:您运行的拳头:
SELECT c_crosstab('Select * from table', 'ct_view', 'usr_id', 'question_id', 'response_value', 'first');
比:
Select * from ct_view;
在contrib/tablefunc/
有一个示例。
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