PostgreSQL的组合，而无需重复

Question

如何使功能，将采取字符串或数组并返回一些长度的所有组合的Postgres？

例如你有ABC，你想用2个字符获得的组合，结果建议立即进行删除是：

AB AC BC

预先感谢您的帮助。

Answer 1

set search_path='tmp'; 

WITH ztab AS (
SELECT idx as idx 
, substring ('WTF!' FROM idx FOR 1) as str 
FROM generate_series(1, char_length('WTF!')) idx 
) 
SELECT t1.str, t2.str 
FROM ztab t1 
JOIN ztab t2 ON t2.idx > t1.idx 
     ; 

结果：

str | str 
-----+----- 
W | T 
W | F 
W | ! 
T | F 
T | ! 
F | ! 
(6 rows) 

可惜我不能找到一种方法，以避免双重字符串常量。 （但是整个事情可以打包成一个函数）如果没有重复的字符（或者你想压制它们），你可以在str而不是idx上进行反连接。

UPDATE（提示从ypercube）看来，OP希望要连接字符串。这样吧::

WITH ztab AS (
SELECT idx as idx 
, substring ('WTF!' FROM idx FOR 1) as str 
FROM generate_series(1, char_length('WTF!')) idx 
) 
SELECT t1.str || t2.str AS results 
FROM ztab t1 
JOIN ztab t2 ON t2.idx > t1.idx 
     ; 

结果：

results 
--------- 
WT 
WF 
W! 
TF 
T! 
F! 
(6 rows) 

UPDATE2：（这里来了递归啄...）

WITH RECURSIVE xtab AS (
     WITH no_cte AS (
     SELECT 
     1::int AS len 
     , idx as idx 
     , substring ('WTF!' FROM idx FOR 1) as str 
     FROM generate_series(1, char_length('WTF!')) idx 
     ) 
     SELECT t0.len as len 
       , t0.idx 
       , t0.str 
     FROM no_cte t0 
     UNION SELECT 1+t1.len 
       , tc.idx 
       , t1.str || tc.str AS str 
     FROM xtab t1 
     JOIN no_cte tc ON tc.idx > t1.idx 
     ) 
SELECT * FROM xtab 
ORDER BY len, str 
-- WHERE len=2 
     ; 

结果3：

len | idx | str 
-----+-----+------ 
    1 | 4 | ! 
    1 | 3 | F 
    1 | 2 | T 
    1 | 1 | W 
    2 | 4 | F! 
    2 | 4 | T! 
    2 | 3 | TF 
    2 | 4 | W! 
    2 | 3 | WF 
    2 | 2 | WT 
    3 | 4 | TF! 
    3 | 4 | WF! 
    3 | 4 | WT! 
    3 | 3 | WTF 
    4 | 4 | WTF! 
(15 rows) 

Answer 2

with chars as (
    select unnest(regexp_split_to_array('ABC','')) as c 
) 
select c1.c||c2.c 
from chars c1 
    cross join chars c2 

要删除您的置换可以使用下列内容：

with chars as (
    select unnest(regexp_split_to_array('ABC','')) as c 
) 
select c1.c||c2.c 
from chars c1 
    cross join chars c2 
where c1.c < c2.c 

Answer 3

如何与多个单词的作品......从@wildplasser从这个源info

WITH RECURSIVE xtab AS (
    WITH no_cte AS (
    SELECT 
    1::int AS len 
    , idx as idx 
    , unnest(ARRAY['MY','POSTGRESQL','VERSION','9.6']) as str 
    FROM generate_series(1, array_length(ARRAY['MY','POSTGRESQL','VERSION','9.6'],1)) idx 
    ) 
    SELECT t0.len as len 
      , t0.idx 
      , t0.str 
    FROM no_cte t0 
    UNION SELECT 1+t1.len 
      , tc.idx 
      , t1.str ||','|| tc.str AS str 
    FROM xtab t1 
    JOIN no_cte tc ON tc.idx > t1.idx 
    ) 
    SELECT distinct 
    array_to_string(ARRAY(SELECT DISTINCT trim(x) FROM unnest(string_to_array(str,',')) x),', ') FROM xtab