如何使用来自 T2 的随机重复行将更大的 table T1 连接到更小的 T2

Question

抱歉，如果已经以某种方式询问过这个问题。我已经搜索过但找不到这个特定的解决方案。所以我将不胜感激答案或指向正确位置的指针... 我有两个不同（变化）长度的表。玩具示例：

T1:

SELECT * FROM T1;
 gid | call_s1 
-----+---------
   1 |       1
   3 |       1
   4 |       1
   7 |       1
   8 |       1
(5 rows)

和

   SELECT * FROM T2;
     gid |       dt_ping       
    -----+---------------------
       1 | 2009-06-06 19:00:00
       2 | 2009-06-06 19:00:15
       3 | 2009-06-06 19:00:30
       4 | 2009-06-06 19:00:45
    (4 rows)

我想得到一个结果 T3，它已将随机行从 T2.dt_ping 分配到 T1 中的每一行，如有必要则重复。例如可能的结果是：

 gid | call_s1  | dt_ping
-----+----------+---------
   1 |       1  | 2009-06-06 19:00:45
   3 |       1  | 2009-06-06 19:00:30
   4 |       1  | 2009-06-06 19:00:15
   7 |       1  | 2009-06-06 19:00:00
   8 |       1  | 2009-06-06 19:00:45

我试过偏移、随机排序等。要么得到笛卡尔积，要么得到空值。例如，这是我最后一次尝试：

SELECT
  T1.gid
  , T1.call_s1
  , T2.dt_ping
FROM
  (
    SELECT
      gid
      , call_s1
      , ceiling(  random() * (SELECT count(*)::int as n FROM fake_called_small )   )  tgid
    FROM fake_called_small
  ) T1
  LEFT OUTER JOIN
  fake_times_small T2
    ON
      T2.gid = T1.tgid;

我得到的结果之一：

 gid | call_s1 |       dt_ping       
-----+---------+---------------------
   1 |       1 | 
   3 |       1 | 2009-06-06 19:00:30
   4 |       1 | 2009-06-06 19:00:45
   7 |       1 | 2009-06-06 19:00:15
   8 |       1 | 
(5 rows)

我知道我遗漏了一些简单的东西，但是什么？

顺便说一句，我试过这个：

SELECT
  T1.gid
  , T1.call_s1
  , (SELECT dt_ping FROM fake_times_small T2 ORDER BY random() LIMIT 1)
FROM fake_called_small  T1;

并得到：

 gid | call_s1 |       dt_ping       
-----+---------+---------------------
   1 |       1 | 2009-06-06 19:00:30
   3 |       1 | 2009-06-06 19:00:30
   4 |       1 | 2009-06-06 19:00:30
   7 |       1 | 2009-06-06 19:00:30
   8 |       1 | 2009-06-06 19:00:30
(5 rows)

重复 dt_ping 的同一行，因为子选择仅完成一次

Answer 1

如果您不关心性能，下面的查询应该可以满足您的要求：

SELECT gid, call_s1, dt_ping
FROM (
  SELECT t1.gid, call_s1, dt_ping,
    ROW_NUMBER() OVER (PARTITION BY t1.gid, call_s1 ORDER BY RANDOM()) AS rn
  FROM t1
  CROSS JOIN t2
) x
WHERE rn = 1;

如何使用来自 T2 的随机重复行将更大的 table T1 连接到更小的 T2

how to join bigger table T1 to a smaller T2 with random repeating rows from T2

random

postgresql

join

postgresql-9.3