[英]I need to join three tables, group the results by one column, and display the highest value from another column
[英]SQL : For each ID, only display the highest value from another column (can't group by)
我這里有這張桌子:
+---------+----------+------------+------------+
| idStep | idProj | dateStart | dateEnd |
+---------+----------+------------+------------+
| 1 | 1 | 2011-07-01 | 2011-09-01 |
| 1 | 2 | 2012-05-01 | 2012-05-10 |
| 1 | 3 | 2011-11-01 | 2012-01-20 |
| 2 | 1 | 2011-09-02 | 2011-11-30 |
| 2 | 2 | 2012-05-11 | 2012-06-01 |
| 2 | 3 | 2012-01-21 | 2012-04-01 |
| 3 | 1 | 2011-12-01 | 2012-07-07 |
| 3 | 2 | 2012-06-02 | 2012-07-01 |
| 3 | 3 | 2012-04-02 | NULL |
| 4 | 1 | 2012-07-08 | NULL |
| 4 | 2 | 2012-07-01 | 2012-07-21 |
| 5 | 2 | 2012-07-22 | 2012-07-23 |
+---------+----------+------------+------------+
我需要通過搜索每個 idProject 的最高 idStep 而不使用 Group By 來找到每個項目的當前步驟,這就是我完全卡住的地方。 如果沒有 GROUP BY,我就無法到達那里。
基本上,輸出應該是這樣的:
+---------+----------+------------+------------+
| idStep | idProj | dateStart | dateEnd |
+---------+----------+------------+------------+
| 3 | 3 | 2012-04-02 | NULL |
| 4 | 1 | 2012-07-08 | NULL |
| 5 | 2 | 2012-07-22 | 2012-07-23 |
+---------+----------+------------+------------+
我想使用這樣構建的查詢
SELECT idProj,idStep
FROM table
WHERE idStep = (SELECT max(idStep) FOR EACH idProj)
我知道 FOR EACH 不是 SQL,我只是想讓我想要的查詢結構可讀。
你想要一個相關的 subuqery:
SELECT idProj, idStep
FROM table t
WHERE t.idStep = (SELECT max(idStep)
FROM table t2
WHERE t2.idProj = t.idProj
);
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