mysql muliple joins from same colums with different keys

问题

I need some help making the second join with the tables below. I got some help from here previously where it was suggested I need to add a second JOIN, however, this is where I'm stuck and need some assistance.

wp-posts

  -----------------
  id | Post_Title | 
  -----------------
  01 | Event 1    | 
  -----------------
  02 | Event 2    | 
  -----------------

wp-postmeta

-------------------------------------------------------
  meta_id  | post_id | meta_key          | meta_value |
  -----------------------------------------------------
  456      | 01      | _EventStartDate   | 01/01/2017 |
  -------------------------------------------------- ---
  789      | 01      | _EventEndDate     | 05/02/2017 |
  -----------------------------------------------------

The end result I'm after is something like;

Title - starts on <_EventStartDate > and ends on <_EventEndDate>

and im using the following to get the data:

    $result = $wpdb->get_results ( "
SELECT $wpdb->posts.ID, $wpdb->posts.post_content, $wpdb->postmeta.meta_id, 

$wpdb->postmeta.post_id, $wpdb->postmeta.meta_key, $wpdb->postmeta.meta_value, $wpdb->posts.post_title 


FROM $wpdb->posts INNER JOIN $wpdb->postmeta ON $wpdb->posts.ID = $wpdb->postmeta.post_id 

WHERE $wpdb->postmeta.meta_key = '_EventStartDate' 

ORDER BY $wpdb->postmeta.meta_value " );

Now I've been told that the WHERE will just return a single row and that I need to make the second JOIN using $wpdb->postmeta.meta_key = '_EventStartDate' but after hours of trying to implement this I'm unable to get and data back at all.

If someone could help solve this it would be immensely helpful as I have a few more queries I would like to write and I'm guessing I will need to use the same principal with them too.

Thanks for reading!!

回答1:

You can do it with a single inner join, grouping by the post ID and then splitting start date and end date:

select 
wpposts.post_content,
substring_index(GROUP_CONCAT(meta_value order by str_to_date(meta_value,'%d/%m/%Y')), ',', 1) as start_date ,
substring_index(GROUP_CONCAT(meta_value order by str_to_date(meta_value,'%d/%m/%Y')), ',', -1) as end_date
from wpposts inner join wppostmeta
on wpposts.id = wppostmeta.post_id
where wppostmeta.meta_key='_EventStartDate' or wppostmeta.meta_key='_EventEndDate'
group by wppostmeta.post_id

回答2:

You can get the start_date and end_date in one row using group by like this:

select
    t1.*,
    max(case when meta_key = '_EventStartDate' then meta_value end) start_date,
    max(case when meta_key = '_EventEndDate' then meta_value end) end_date
from wp_posts t1
inner join wp_postmeta t2
on t1.id = t2.post_id
group by t1.post_id;

if ONLY_FULL_GROUP_BY is enabled, change the group by clause to:

group by t1.post_id, t1.Post_title;

回答3:

Assuming there is only one _EventStartDate and one _EventEndDate per post in wp-postmeta, a simplified query would be:

SELECT p.post_title, pm1.meta_value AS start_date, pm2.meta_value AS end_date 
FROM wp-posts p 
     INNER JOIN wp-postmeta pm1
             ON  p.post_id     = pm1.post_id
            AND  pm1.meta_ekey = '_EventStartDate'  
     INNER JOIN wp-postmeta pm2
             ON  p.post_id     = pm2.post_id
            AND  pm2.meta_ekey = '_EventEndDate'

来源：https://stackoverflow.com/questions/41891324/mysql-muliple-joins-from-same-colums-with-different-keys