LEFT JOIN訂單和限制

Question

這是我的查詢：

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`)

我的問題是這個左連接：

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

因為，如果a有很多行，它會返回像我的情況：

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

我希望它選擇具有級別desc和限制1的order的a.name ，因此它將返回更高level的名稱(a.account_id = fr.author) 。

希望你能得到我。 如果沒有，請隨時發表評論。

Answer 1

嘗試更換：

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

有：

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
       )

Answer 2

用以下內容替換LEFT JOIN子句：

...
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)
...

如果ps有多個行具有相同的帳戶ID且具有相同級別且該級別是最高級別，則仍會返回多行：

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

如果出現這種情況，那么您必須決定要做什么 - 並適當地調整查詢。 例如，您可能決定使用帶有GROUP BY子句的MAX(b.name)來隨意選擇兩個名稱中的字母順序。