仅从具有 where 条件的连接表中选择第一条记录

Question

我已经使用 PHP 解决了这个问题，但我知道如果我可以查询它会更快。我有以下 2 个表。

projects:     
+------------+--------------+ 
| project_id | project_name | 
+------------+--------------+ 
|  1         | a            | 
|  2         | b            | 
|  3         | c            | 
|  4         | d            | 
+------------+--------------+ 

tasks:
+------------+------------+-------------+---------------------+ 
| project_id |  task_id   | task_status |  task_due_timestamp |
+------------+------------+-------------+---------------------+ 
|      1     |      1     |     1       | 2015-01-01 12:00:00 |
|      1     |      2     |     2       | 2015-01-02 12:00:00 |
|      2     |      3     |     3       | 2015-01-03 12:00:00 |
|      3     |      4     |     1       | 2015-01-04 12:00:00 |
+------------+------------+-------------+---------------------+

我需要一个查询来选择所有项目及其关联任务的状态为 1 或 2。它选择的任务必须是最新的。见下表。

records after query:
+------------+--------------+------------+-------------+---------------------+ 
| project_id | project_name |  task_id   | task_status |  task_due_timestamp |
+------------+--------------+------------+-------------+---------------------+ 
|     1      | a            |      1     |     1       | 2015-01-01 12:00:00 |
|     2      | b            |     null   |    null     |       null          |
|     3      | c            |      3     |     3       | 2015-01-01 12:00:00 |
|     4      | d            |      null  |     null    | null                |
+------------+--------------+------------+-------------+---------------------+

如您所见，项目 2 不应该有任务，因为任务状态为 2。项目 4 没有项目。项目 1 有 2 个任务，但最早的任务提前了一天。

任何帮助将不胜感激。

Answer 1

您可以尝试对状态为1 或2 的最新任务进行左连接。查询会是这样的：

select p.*, t.*
from projects as p
  left join (
    select t1.*
    from tasks as t1
    where t1.task_status in (1, 2)
      and not exists (
        select task_id from tasks as t2
        where t1.task_id <> t2.task_id
          and t1.project_id = t2.project_id
          and t2.task_status in (1, 2)
          and t2.task_due_timestamp > t1.task_due_timestamp
      )
  ) as t on p.project_id = t.project_id

Live demo

Answer 2

你可以试试这个：-

SELECT P.project_id, P.project_name, T.task_id, T.task_status, MIN(T.task_due_timestamp)
FROM projects P INNER JOIN tasks T 
ON P.project_id = T.project_id 
AND T.task_status IN (1,2)
GROUP BY P.project_id, P.project_name, T.task_id, T.task_status