计算与MySQL中每个唯一cRt的前一行的日期差

Question

我有一个表，其列为cDate。

我想计算具有相同cRt值的两个连续记录之间的时间差。

例如：

+---------------+----------+---------------------+-----------------------------+----------+
| cNode         | cEl      | cDate               | cEvent                      | cRt      |
+---------------+----------+---------------------+-----------------------------+----------+
| SD20-2-100102 | SD100102 | 2020-04-13 11:34:35 | POSITIVE REMOTE CONTROL SN  | SD100102 |
| SD20-2-100102 | SD100102 | 2020-04-13 07:15:14 | NEGATIVE REMOTE CONTROL SN  | SD100102 |
+---------------+----------+---------------------+-----------------------------+----------+

SD100102 cRt的时差是259。

mysql> 
SELECT
    ROUND(
        time_to_sec(
            (
                TIMEDIFF(
                    '2020-04-13 11:34:35',
                    '2020-04-13 07:15:14'
                )
            )
        ) / 60
    ) AS td;
+-----+
| td  |
+-----+
| 259 |
+-----+
1 row in set

表格：

+---------------+----------+---------------------+-----------------------------+----------+
| cNode         | cEl      | cDate               | cEvent                      | cRt      |
+---------------+----------+---------------------+-----------------------------+----------+
| SD20-2-100102 | SD100102 | 2020-04-13 11:34:35 | POSITIVE REMOTE CONTROL SN  | SD100102 |
| SD20-2-100102 | SD100102 | 2020-04-13 07:15:14 | NEGATIVE REMOTE CONTROL SN  | SD100102 |
| SD40-2-103423 | SD103423 | 2020-04-06 06:21:50 | POSITIVE REMOTE CONTROL SN  | SD103423 |
| SD40-2-103423 | SD103423 | 2020-04-06 06:20:21 | NEGATIVE REMOTE CONTROL SN  | SD103423 |
| SD20-2-103484 | SD103484 | 2020-04-23 15:19:37 | POSITIVE REMOTE CONTROL SN  | SD103484 |
| SD20-2-103484 | SD103484 | 2020-04-23 15:17:53 | NEGATIVE REMOTE CONTROL SN  | SD103484 |
| SD20-2-103484 | SD103484 | 2020-04-21 14:29:50 | POSITIVE REMOTE CONTROL SN  | SD103484 |
| SD20-2-103484 | SD103484 | 2020-04-21 14:19:50 | NEGATIVE REMOTE CONTROL SN  | SD103484 |
| SD20-2-103484 | SD103484 | 2020-02-13 07:33:00 | POSITIVE REMOTE CONTROL SN  | SD103484 |
| SD20-2-103484 | SD103484 | 2020-02-13 07:31:55 | NEGATIVE REMOTE CONTROL SN  | SD103484 |
| SD20-2-103484 | SD103484 | 2020-01-13 15:57:16 | POSITIVE REMOTE CONTROL SN  | SD103484 |
| SD20-2-103484 | SD103484 | 2020-01-12 00:10:50 | NEGATIVE REMOTE CONTROL SN  | SD103484 |
| SD40-2-103565 | SD103565 | 2020-04-07 12:20:48 | POSITIVE REMOTE CONTROL SN  | SD103565 |
| SD40-2-103565 | SD103565 | 2020-04-07 11:59:47 | NEGATIVE REMOTE CONTROL SN  | SD103565 |
| SD40-2-103565 | SD103565 | 2020-03-05 11:46:03 | POSITIVE REMOTE CONTROL SN  | SD103565 |
| SD40-2-103565 | SD103565 | 2020-03-05 09:13:46 | NEGATIVE REMOTE CONTROL SN  | SD103565 |
| SD40-2-103565 | SD103565 | 2020-02-18 12:30:21 | POSITIVE REMOTE CONTROL SN  | SD103565 |
| SD40-2-103565 | SD103565 | 2020-02-18 12:29:12 | NEGATIVE REMOTE CONTROL SN  | SD103565 |
| SD40-2-103565 | SD103565 | 2020-02-14 07:52:28 | POSITIVE REMOTE CONTROL SN  | SD103565 |
| SD40-2-103565 | SD103565 | 2020-02-14 07:50:42 | NEGATIVE REMOTE CONTROL SN  | SD103565 |
+---------------+----------+---------------------+-----------------------------+----------+

谢谢。

Answer 1

在8.0之前的MySql版本中，没有窗口函数，没有执行此操作的高性能方法。一种方法是让每行在相同cRt的当前cDate之前获取最大cDate：

SELECT t.*,
  ROUND(time_to_sec((TIMEDIFF(
    t.cDate,
    (SELECT MAX(cDate) FROM tablename WHERE cRt = t.cRt AND cDate < t.cDate)
  ))) / 60) AS td
FROM tablename t