MySQL左联接返回只返回一行

Question

我正在尝试从历史航班表中查找先前的航班，我只有1个飞机的历史表机尾号和计划的日期偏离，我想用该飞机先前的航班信息创建一个新表，下面是示例桌子的

原始表：

|flight number|scheduled departure flight|
| A           | 1 Jan 2018 10:00         |
| A           | 1 Jan 2018 19:00         |
| B           | 1 Jan 2018 11:00         |
| B           | 1 Jan 2018 19:00         |
| B           | 1 Jan 2018 21:00         |

预期结果：

|flight number|scheduled departure flight| previous scheduled departure |
| A           | 1 Jan 2018 10:00         | NULL                         |
| A           | 1 Jan 2018 19:00         | 1 Jan 10:00                  |
| B           | 1 Jan 2018 11:00         | NULL                         |
| B           | 1 Jan 2018 19:00         | 1 Jan 2018 11:00             |
| B           | 1 Jan 2018 21:00         | 1 jan 2018 19:00             |

我已经做了一天了，感谢您的任何帮助

Answer 1

您正在寻找LAG窗口功能，但支持高于8.0的mysql版本

您可以尝试使用子查询来创建lag窗口函数。

模式（MySQL v5.7）

查询＃1

SELECT `flight number`,
       `scheduled departure flight`,
       (select `scheduled departure flight` 
       from T tt
       where t1.`scheduled departure flight` > tt.`scheduled departure flight`
       AND t1.`flight number` = tt.`flight number`
       order by tt.`scheduled departure flight` desc
       limit 1
       ) 'previous scheduled departure'
FROM T t1;

| flight number | scheduled departure flight | previous scheduled departure |
| ------------- | -------------------------- | ---------------------------- |
| A             | 2018-06-01 10:00:00        |                              |
| A             | 2018-06-01 19:00:00        | 2018-06-01 10:00:00          |
| B             | 2018-06-01 11:00:00        |                              |
| B             | 2018-06-01 19:00:00        | 2018-06-01 11:00:00          |
| B             | 2018-06-01 21:00:00        | 2018-06-01 19:00:00          |

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在数据库小提琴上查看

如果您的mysql版本支持LAG ，则可以尝试此操作。

模式（MySQL v8.0）

CREATE TABLE T(
   `flight number` varchar(5),
    `scheduled departure flight` datetime
);



INSERT INTO T VALUES ('A','2018-06-01 10:00');
INSERT INTO T VALUES ('A','2018-06-01 19:00');
INSERT INTO T VALUES ('B','2018-06-01 11:00');
INSERT INTO T VALUES ('B','2018-06-01 19:00');
INSERT INTO T VALUES ('B','2018-06-01 21:00');

---

查询＃1

SELECT `flight number`,
       `scheduled departure flight`,
       LAG(`scheduled departure flight`) OVER(PARTITION BY  `flight number` ORDER BY `scheduled departure flight`) 'previous scheduled departure'
FROM T t1;

---

在数据库小提琴上查看