如何將此SQL轉換為無需子查詢的左聯接？

Question

我想在mysql中創建視圖，但是在mysql中不支持子查詢。 如何編寫不帶子查詢的sql？

select * from dev_location t1

        inner join 

    (
        select
            `dev_location`.`device_id` AS `device_id`,
            max(`dev_location`.`id`) AS `id`
        from
            `dev_location`
        group by `dev_location`.`device_id`) t2

    on t1.id = t2.id

Answer 1

MySQL視圖在from子句中不支持子查詢。 以下應該在視圖中起作用：

select dl.*
from dev_location dl
where not exists (select 1
                  from dev_location dl2
                  where dl2.device_id = dl.device_id and
                        dl2.id > dl.id
                 );

這將查詢重新dev_location為：“從dev_location中獲取device_id沒有更大id所有行。” 這是獲取最大值的尷尬方式。

並且，在dev_location(device_id, id)上使用索引，它的性能可能比您的版本更好。