如何计算 mysql 中一组中的记录数

Question

比如我有一个table1:

+---------------------+
| StartTime           |
+---------------------+
| 2014-04-09 09:47:53 |
| 2014-04-09 09:47:53 |
| 2014-04-09 09:47:55 |
| 2014-04-09 09:47:56 |
| 2014-04-09 09:47:57 |
| 2014-04-09 09:47:59 |
+---------------------+

表没有连续的时间。
例如，它没有

2014-04-09 09:47:54,
2014-04-09 09:47:58

等然后我使用：

select count(*),StartTime from this_tables group by StartTime;

我得到另一个表2：

+-------+---------------------+
| count | StartTime           |
+-------+---------------------+
|     2 | 2014-04-09 09:47:53 |
|     1 | 2014-04-09 09:47:55 |
|     1 | 2014-04-09 09:47:56 |
|     1 | 2014-04-09 09:47:57 |
|     1 | 2014-04-09 09:47:59 |
+-------+---------------------+

但我想要一个 table3 看起来像：

+-------+---------------------+
| count | StartTime           |
+-------+---------------------+
|     2 | 2014-04-09 09:47:53 |
|     0 | 2014-04-09 09:47:54 |
|     1 | 2014-04-09 09:47:55 |
|     1 | 2014-04-09 09:47:56 |
|     1 | 2014-04-09 09:47:57 |
|     0 | 2014-04-09 09:47:58 |
|     1 | 2014-04-09 09:47:59 |
+-------+---------------------+

Table 3 可以为不存在的 StartTime 包含“0”。 mysql怎么办？

Answer 1

你需要一个数字（计数）table。您可以通过以下方式创建和填充此类 table（在本例中为 100 行）

CREATE TABLE tally (n INT NOT NULL AUTO_INCREMENT PRIMARY KEY);

INSERT INTO tally
SELECT a.N + b.N * 10 + 1 n
 FROM 
(SELECT 0 AS N UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3 UNION ALL SELECT 4 UNION ALL SELECT 5 UNION ALL SELECT 6 UNION ALL SELECT 7 UNION ALL SELECT 8 UNION ALL SELECT 9) a
,(SELECT 0 AS N UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3 UNION ALL SELECT 4 UNION ALL SELECT 5 UNION ALL SELECT 6 UNION ALL SELECT 7 UNION ALL SELECT 8 UNION ALL SELECT 9) b
ORDER BY n;

您需要在此 table 中拥有与您查询的最大可能间隔中的秒数一样多的行。

现在您正在寻找的查询（为了这个例子的一天 2014-04-09）可能看起来像这样

SELECT b.starttime, COALESCE(count, 0) count
  FROM
(
  SELECT min_dt + INTERVAL n-1 SECOND starttime
    FROM tally t CROSS JOIN 
  (
    SELECT MIN(starttime) min_dt, MAX(starttime) max_dt
      FROM table1
     WHERE starttime >= '2014-04-09'
       AND starttime < '2014-04-09' + INTERVAL 1 DAY
  ) i 
   WHERE t.n-1 <= TIMESTAMPDIFF(SECOND, min_dt, max_dt)
) b LEFT JOIN 
(
  SELECT starttime, COUNT(*) count
    FROM table1
   WHERE starttime >= '2014-04-09'
     AND starttime < '2014-04-09' + INTERVAL 1 DAY
   GROUP BY starttime
) q
    ON b.starttime = q.starttime

输出：

|                    STARTTIME | COUNT |
|------------------------------|-------|
| April, 09 2014 09:47:53+0000 |     2 |
| April, 09 2014 09:47:54+0000 |     0 |
| April, 09 2014 09:47:55+0000 |     1 |
| April, 09 2014 09:47:56+0000 |     1 |
| April, 09 2014 09:47:57+0000 |     1 |
| April, 09 2014 09:47:58+0000 |     0 |
| April, 09 2014 09:47:59+0000 |     1 |

这是一个SQLFiddle演示

如何计算 mysql 中一组中的记录数

how to count the num of records in a group in mysql

mysql

count