如何将子查询替换为联接查询

Question

我正在使用此查询

select t.task_id,td.task_detail_id ,
(SELECT COUNT(1) FROM task_detail tdd where tdd.task_id=t.task_id and  
tdd.status='ACTIVE' ) as total
from task t inner join task_detail td  on td.task_id=t.task_id 
where td.task_id=t.task_id and td.status='ACTIVE' ; 

查询正在以预期的结果运行，但是我想将total column子查询更改为联接。

Answer 1

请您试试这个-

SELECT 
t.task_id, 
td.task_detail_id, 
COUNT(*)
FROM task t
INNER JOIN task_detail td 
    ON td.task_id = t.task_id AND td.STATUS = 'ACTIVE'
GROUP BY t.task_id, td.task_detail_id

Answer 2

在MySQL 8+中，您将使用窗口函数

select t.task_id, td.task_detail_id,
       count(*) over (partition by t.task_id) as total
from task t inner join
     task_detail td
     on td.task_id = t.task_id 
where td.status = 'ACTIVE' ; 

在早期版本中，建议使用子查询，但是您可以将其移动到from子句：

select t.task_id, td.task_detail_id,
       td2.total
from task t inner join
     task_detail td
     on td.task_id = t.task_id join
     (select count(*) as total
      from task_detail td2
      where td2.status = 'ACTIVE'
      group by td2.task_id
     ) td2
     on td2.task_id = td.task_id
where td.status = 'ACTIVE' ;