Question

在我的数据库中，一个主管可以有很多任务，因此任务表有一个taskable_id和taskable_type，其中taskable_id是引导的id，taskable_type是“Lead”，遵循Ruby on Rails多态约定。

我想为所有预约状态为1的潜在客户选择第一项任务。就是这样。结果集应该是一组任务。这就是我想出的：

SELECT 
    `tasks`.* 
FROM 
    `tasks`  
      WHERE `tasks`.`taskable_id` IN (1, 2) 
      AND 
      `tasks`.`taskable_type` = 'Lead' 
      AND 
      `tasks`.`id` 
      IN 
        (
            SELECT  `tasks`.`id` 
            FROM `tasks`  
            WHERE `tasks`.`taskable_id` IN (1, 2) 
            AND 
            `tasks`.`taskable_type` = 'Lead' 
            AND 
            'appointment_status' = 1 
            ORDER BY created_at asc 
            LIMIT 1
        );

但是这给出了错误“这个版本的MySQL还不支持'LIMIT＆amp; IN / ALL / ANY / SOME子查询”。其他stackoverflow帖子说，对于这个错误，LIMIT必须在子查询之外。

以下是示例数据：

mysql> SELECT * FROM tasks;
+----+------+--------+-------------+----------+---------------------+---------------------+-----------+---------------------+---------------------+---------------+-------------+
| id | name | status | description | priority | created_at          | updated_at          | task_type | date_start          | date_due            | taskable_type | taskable_id |
+----+------+--------+-------------+----------+---------------------+---------------------+-----------+---------------------+---------------------+---------------+-------------+
|  1 | NULL |      2 | NULL        |        1 | 2014-09-24 17:53:58 | 2014-09-24 17:53:58 |         1 | 2014-09-25 11:30:00 | 2014-09-25 12:00:00 | Lead          |           1 |
|  2 | NULL |      2 | NULL        |        1 | 2014-09-25 12:45:50 | 2014-09-25 12:45:50 |         1 | 2014-09-25 12:45:00 | 2014-09-25 13:15:00 | Lead          |           2 |

mysql> SELECT id, appointment_status, created_at, updated_at FROM leads;
+----+--------------------+---------------------+---------------------+
| id | appointment_status | created_at          | updated_at          |
+----+--------------------+---------------------+---------------------+
|  1 |                  1 | 2014-09-24 17:45:12 | 2014-09-24 17:53:58 |
|  2 |                  1 | 2014-09-25 12:45:46 | 2014-09-25 12:45:50 |

Answer 1

经过一些实验，我觉得它有效：

SELECT 
    tasks.* 
FROM
    tasks 
    INNER JOIN 
        leads
        ON leads.id = tasks.taskable_id AND tasks.taskable_type = "Lead"
WHERE 
    tasks.taskable_id IN (1, 2) 
    AND 
    (
        tasks.id = 
        (
            SELECT tasks.id 
            FROM tasks
            WHERE tasks.taskable_id = leads.id 
            AND
            tasks.taskable_type = "Lead"
            AND 
            appointment_status = 1
            ORDER BY created_at asc
            LIMIT 1
        )
    )

Answer 2

没有uservars的方法：

SELECT
    tasks.*
FROM
    (SELECT taskable_id,
        MIN(t.created_at) created_at
    FROM
        tasks t
    JOIN
        leads l USING (taskable_id)
    WHERE
        taskable_id IN (1,2)
        AND t.taskable_type = 'Lead'
        AND l.appointment_status = 1
    GROUP BY taskable_id
    ) x
JOIN
    tasks
    USING (taskable_id, created_at)

在Mysql中使用LIMIT和子查询

2 个答案: