如何使用GROUP BY和ORDER BY关键字从表中获取数据？

Question

I have following tables with columns :

1) users

user_id  username

2) projects

p_id   p_name

3) project_status

psdi    p_id    cdid   cid   sid  short_list   res_sent   status_date

4) company

cid   company_name

5) status

sid    status_name    status_order    is_cv_sent 

6) projects_log

pl_id   cdid    project_name     p_id   user_id   status   date_time    

Now, in this projects_log table there are same p_id value exist.

I want to show all unique p_id with latest date_time. as DESC order where pl.cdid = $cdid

I am using following query but can't get the results. It's not showing me all unique p_id as date_time DESC order :(

$get_log = mysqli_query($link, "SELECT 
    pl.*, 
    u.username, 
    p.p_name, 
    c.company_name, 
    s.status_name, 
    ps.* 
    FROM projects_log AS pl 
    LEFT JOIN users AS u ON u.user_id =  pl.user_id 
    LEFT JOIN projects AS p ON p.p_id = pl.p_id     
    LEFT JOIN project_status AS ps ON ps.p_id = pl.p_id 
    LEFT JOIN company AS c ON c.cid = ps.cid 
    LEFT JOIN status AS s ON s.sid = ps.sid 
    WHERE pl.cdid = '$cdid' GROUP BY pl.p_id
    ORDER BY pl.pl_id DESC ");

Answer 1

Using below query you will able to achieve group by p_id and will able to get values of max date_time row.

$get_log = mysqli_query($link, "SELECT 
    pl.*, 
    u.username, 
    p.p_name, 
    c.company_name, 
    s.status_name, 
    ps.* 
    FROM (
        select MAX(date_time) as MaxDateTime,p_id from projects_log group by p_id
    ) as mpl join projects_log AS pl on mpl.MaxDateTime = pl.date_time and mpl.p_id = pl.p_id
    LEFT JOIN users AS u ON u.user_id =  pl.user_id 
    LEFT JOIN projects AS p ON p.p_id = pl.p_id     
    LEFT JOIN project_status AS ps ON ps.p_id = pl.p_id 
    LEFT JOIN company AS c ON c.cid = ps.cid 
    LEFT JOIN status AS s ON s.sid = ps.sid 
    WHERE pl.cdid = '$cdid'
    ORDER BY pl.pl_id DESC ");