CODEWITHSUNDEEP
mysqljoinleft-joinsql-order-by
Cyclone
Cyclone

Reputation: 15279

LEFT JOIN order and limit

This is my query:

SELECT `p`.`name` AS 'postauthor', `a`.`name` AS 'authorname',
       `fr`.`pid`, `fp`.`post_topic` AS 'threadname', `fr`.`reason`
  FROM `z_forum_reports` `fr`
  LEFT JOIN `forums` `f` ON (`f`.`id` = `fr`.`pid`)
  LEFT JOIN `forums` `fp` ON (`f`.`first_post` = `fp`.`id`) 
  LEFT JOIN `ps` `p` ON (`p`.`id` = `f`.`author_guid`)
  LEFT JOIN `ps` `a` ON (`a`.`account_id` = `fr`.`author`)

My problem is this left join:

SELECT `a`.`name`, `a`.`level`
[..]
LEFT JOIN `ps` `a` ON (`a`.`account_id` = `fr`.`author`)

Since, in case a has MANY rows and it'll return like in my case:

NAME  | LEVEL
Test1 | 1
Test2 | 120
Test3 | 2
Test4 | 1

I want it to select a.name with order of level desc and limit 1, so it'll return the name of higher level where (a.account_id = fr.author).

Hope you got me. If not, feel free to post a comment.

Upvotes: 22

Views: 46731

Answers (2)

ypercubeᵀᴹ
ypercubeᵀᴹ

Reputation: 115630

Try replacing:

LEFT JOIN ps a ON a.account_id = fr.author

with:

LEFT JOIN ps a 
  ON a.PrimaryKey                         --- the Primary Key of ps
     = ( SELECT b.PrimaryKey 
         FROM ps AS b 
         WHERE b.account_id = fr.author
         ORDER BY b.level DESC
         LIMIT 1
       )

Upvotes: 45

Jonathan Leffler
Jonathan Leffler

Reputation: 755006

Replace the LEFT JOIN clause with something like:

...
LEFT JOIN (SELECT b.account_id, b.name
             FROM (SELECT c.account_id, MAX(c.level) AS level
                     FROM ps AS c
                    GROUP BY c.account_id) AS d
             JOIN ps AS b ON b.account_id = d.account_id AND b.level = d.level
          ) AS a
       ON (a.account_id = fr.author)
...

This will still return multiple rows if there were several rows in ps with the same account ID and the same level and that level was the maximum level:

NAME  | LEVEL
Test1 | 1
Test2 | 120
Test3 | 2
Test4 | 1
Test5 | 120

If this situation can arise, then you have to decide what you want to do - and tune the query appropriately. For example, you might decide to use MAX(b.name) with a GROUP BY clause to arbitrarily select the alphabetically later of the two names.

Upvotes: 2

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