[英]SQL: self JOIN top 1 from table SELECTs? (or better way of doing it…)
這可能之前已經討論過(或者太簡單了),但是我找不到一個簡單的答案:應該怎么做:選擇(單個)最早的日期和每個id的最近日期?
t1: ==?==> (earliest) (most recent)
|id|date(DESC)| |id|future_date | past_date |
+==+==========+ +==+=============+===============+
|1 | d1 | | 1| d1 | d3 |
|2 | d2 | | 2| d2 | d6 |
(<==now) | 3| | d4 |
|1 | d3 |
|3 | d4 |
|1 | d5 |
|2 | d6 |
我按照以下思路進行思考,但是我感覺這是相當復雜的/錯誤的語法,而且我還沒有弄清楚如何限制單個結果(即最近的/最近的)。 有什么建議么?
SELECT t_1.id,t_1.date AS future_date,t_2.date AS past_date
FROM (SELECT * FROM t1 WHERE t1.date>CURRENT_TIMESTAMP) t_1
LEFT OUTER JOIN
(SELECT TOP 1 * FROM t1 WHERE t1.date<CURRENT_TIMESTAMP) t_2
ON t_1.id=t_2.id
我認為您可以使用:
SELECT x.id,
MIN(y.date) AS future_date,
MAX(z.date) AS past_date
FROM (SELECT DISTINCT t.id
FROM YOUR_TABLE t) x
LEFT JOIN YOUR_TABLE y ON y.id = x.id
AND y.date > CURRENT_TIMESTAMP
LEFT JOIN YOUR_TABLE z ON z.id = x.id
AND z.date < CURRENT_TIMESTAMP
GROUP BY x.id
目前尚不清楚您正在使用的數據庫-TOP僅是TSQL / SQL Server語法(2000+),而MySQL和PostgreSQL支持NOW as NOW()
... CURRENT_TIMESTAMP是ANSI,並且受所有人支持。
為什么不行呢?
select min(date) past_date,max(DATE)future_date
from t1
HAVING MIN(date) < CURRENT_TIMESTAMP AND MAX(DATE) < CURRENT_TIMESTAMP
group by id
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