Question

我有下表

CREATE TABLE temp (
    id SERIAL,
    other_id INTEGER NOT NULL, -- some ForeignKey
    date DATE NOT NULL
)

我希望通过具有相同date的上一个（最近的）other_id项将此表连接到自身。像

这样的东西

SELECT count(*) 
FROM temp AS t1
JOIN temp AS t2 ON (t2.other_id = t1.other_id AND t2.date < t1.date)

但t2.date必须最接近t1.date（不是更低的日期）。

这可能吗？

Answer 1

您可以使用如下查询：

WITH temp_rn AS (
  SELECT id, other_id, date,
         ROW_NUMBER() OVER (PARTITION BY other_id 
                            ORDER BY date) AS rn
  FROM temp
)
SELECT t1.*
FROM temp_rn AS t1
LEFT JOIN temp_rn AS t2 ON t1.other_id = t2.other_id AND t1.rn = t2.rn + 1

查询使用ROW_NUMBER来检测“上一个”行：它是在同一个other_id切片中具有前一个行号的行。

Answer 2

你所追求的并不完全清楚，但是这样的事情可能会这样做：

select count(*)
from temp as t1
  join lateral (
      select t.other_id, max(t.date)
      from temp as t 
      where t.date < t1.date
        and t.other_id = t1.other_id
      group by t2.other_id
  ) as t2 on t2.other_id = t1.other_id;

Postgres：如何从同一个表中加入最接近的值

2 个答案: