仅从具有where条件的联接表中选择第一条记录

Question

I have worked this problem out using PHP but I know it will be faster if I could make one query for it. I have the following 2 tables.

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 |
+------------+------------+-------------+---------------------+

I need a query that will select ALL projects with their associated task that has a status of 1 OR 2. And the task it selects has to be the most recent one. See the below table.

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                |
+------------+--------------+------------+-------------+---------------------+

As you can see project 2 shouldn't have a task as the task status is 2. And project 4 has no projects. Project 1 has 2 tasks but the earliest one is earlier by a day.

Any help would be greatly appreciated.

Answer 1

You can try with a left join on the most recent tasks with status 1 or 2 . The query would like something like this:

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

You can try this:-

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