带有元素编号的PostgreSQL unnest()

Question

当我有一个具有分隔值的列时,我可以使用该unnest()函数:

myTable
id | elements
---+------------
1  |ab,cd,efg,hi
2  |jk,lm,no,pq
3  |rstuv,wxyz

select id, unnest(string_to_array(elements, ',')) AS elem
from myTable

id | elem
---+-----
1  | ab
1  | cd
1  | efg
1  | hi
2  | jk
...

我如何包含元素编号？即:

id | elem | nr
---+------+---
1  | ab   | 1
1  | cd   | 2
1  | efg  | 3
1  | hi   | 4
2  | jk   | 1
...

我想要源字符串中每个元素的原始位置.我试着窗口函数(row_number(),rank()等等),但我总是得到1.也许是因为它们位于源表的同一行？

我知道这是一个糟糕的桌面设计.这不是我的,我只是想解决它.

Answer 1

Postgres 9.4或更高版本

使用WITH ORDINALITY了一组返回功能:

当FROM子句中的函数以后缀为后缀时WITH ORDINALITY,bigint会在输出中附加一列,该 列从1开始,并对函数输出的每一行递增1.这在设置返回函数的情况下最有用,例如UNNEST().

结合LATERALpg 9.3+中的功能,并根据pgsql-hackers上的这个帖子,上面的查询现在可以写成:

SELECT t.id, a.elem, a.nr
FROM   tbl AS t
LEFT   JOIN LATERAL unnest(string_to_array(t.elements, ','))
                    WITH ORDINALITY AS a(elem, nr) ON TRUE;

LEFT JOIN ... ON TRUE保留左表中的所有行,即使右表中的表达式不返回任何行.如果没关系,你可以使用这个等效的,不那么详细的形式与隐含CROSS JOIN LATERAL:

SELECT t.id, a.elem, a.nr
FROM   tbl t, unnest(string_to_array(t.elements, ',')) WITH ORDINALITY a(elem, nr);

或者更简单,如果基于实际数组(arr作为数组列):

SELECT t.id, a.elem, a.nr
FROM   tbl t, unnest(t.arr) WITH ORDINALITY a(elem, nr);

甚至,使用最少的语法:

SELECT id, a, ordinality
FROM   tbl, unnest(arr) WITH ORDINALITY a;

a是自动表和列别名.添加的常规列的默认名称是ordinality.但是添加显式列别名和表限定列会更好(更安全,更清晰).

Postgres 8.4 - 9.3

随着row_number() OVER (PARTITION BY id ORDER BY elem)您根据排序顺序,不是的序数得到的数字原顺序位置的的字符串中.

你可以简单地省略ORDER BY:

SELECT *, row_number() OVER (PARTITION by id) AS nr
FROM  (SELECT id, regexp_split_to_table(elements, ',') AS elem FROM tbl) t;

虽然这通常有效,但我从未在简单的查询中看到过它,PostgreSQL没有关于行的顺序断言ORDER BY.由于实现细节,它恰好起作用.

要保证以空格分隔的字符串中的元素的序数:

SELECT id, arr[nr] AS elem, nr
FROM  (
   SELECT *, generate_subscripts(arr, 1) AS nr
   FROM  (SELECT id, string_to_array(elements, ' ') AS arr FROM tbl) t
   ) sub;

或者如果基于实际数组更简单:

SELECT id, arr[nr] AS elem, nr
FROM  (SELECT *, generate_subscripts(arr, 1) AS nr FROM tbl) t;

关于dba.SE的相关回答:

• 如何保持unnested数组中元素的原始顺序？

Postgres 8.1 - 8.4

这些功能都不是可用的,但:RETURNS TABLE,generate_subscripts(),unnest(),array_length().
但这有效:

CREATE FUNCTION f_unnest_ord(anyarray, OUT val anyelement, OUT ordinality integer)
  RETURNS SETOF record LANGUAGE sql IMMUTABLE AS
'SELECT $1[i], i - array_lower($1,1) + 1
 FROM   generate_series(array_lower($1,1), array_upper($1,1)) i';

特别要注意,数组索引可以与元素的顺序位置不同.考虑这个具有扩展功能的演示:

CREATE FUNCTION f_unnest_ord_idx(anyarray, OUT val anyelement, OUT ordinality int, OUT idx int)
  RETURNS SETOF record  LANGUAGE sql IMMUTABLE AS
'SELECT $1[i], i - array_lower($1,1) + 1, i
 FROM   generate_series(array_lower($1,1), array_upper($1,1)) i';

SELECT id, arr, (rec).*
FROM  (
   SELECT *, f_unnest_ord_idx(arr) AS rec
   FROM  (VALUES (1, '{a,b,c}'::text[])  --  short for: '[1:3]={a,b,c}'
               , (2, '[5:7]={a,b,c}')
               , (3, '[-9:-7]={a,b,c}')
      ) t(id, arr)
   ) sub;

 id |       arr       | val | ordinality | idx
----+-----------------+-----+------------+-----
  1 | {a,b,c}         | a   |          1 |   1
  1 | {a,b,c}         | b   |          2 |   2
  1 | {a,b,c}         | c   |          3 |   3
  2 | [5:7]={a,b,c}   | a   |          1 |   5
  2 | [5:7]={a,b,c}   | b   |          2 |   6
  2 | [5:7]={a,b,c}   | c   |          3 |   7
  3 | [-9:-7]={a,b,c} | a   |          1 |  -9
  3 | [-9:-7]={a,b,c} | b   |          2 |  -8
  3 | [-9:-7]={a,b,c} | c   |          3 |  -7

相比:

• 规范化1维数组的数组下标,使它们从1开始

Answer 2

尝试:

select v.*, row_number() over (partition by id order by elem) rn from
(select
    id,
    unnest(string_to_array(elements, ',')) AS elem
 from myTable) v

Answer 3

使用下标生成函数.
http://www.postgresql.org/docs/current/static/functions-srf.html#FUNCTIONS-SRF-SUBSCRIPTS

例如:

SELECT 
  id
  , elements[i] AS elem
  , i AS nr
FROM
  ( SELECT 
      id
      , elements
      , generate_subscripts(elements, 1) AS i
    FROM
      ( SELECT
          id
          , string_to_array(elements, ',') AS elements
        FROM
          myTable
      ) AS foo
  ) bar
;

更简单:

SELECT
  id
  , unnest(elements) AS elem
  , generate_subscripts(elements, 1) AS nr
FROM
  ( SELECT
      id
      , string_to_array(elements, ',') AS elements
    FROM
      myTable
  ) AS foo
;