Question

这是我的问题：

SELECT e.id, (SELECT MIN(u.id) id
        FROM (SELECT MIN(id) id
            FROM events
            WHERE author_id = 32
            GROUP BY type, post_id, table_code, comment_id, context
            ORDER BY MIN(id) desc 
            LIMIT 15) as u
        ) as indicator_id
FROM events e
WHERE author_id = 32
HAVING e.id >= indicator_id
ORDER BY id DESC

它也有效，并返回两列：id和indicator_id。

我需要得到id。如何省略indicator_id？如您所见，我需要将indicator_id用于HAVING子句。所以我不能省略整个子查询。我需要将它移到除SELECT语句之外的其他地方。语法是什么？

Answer 1

您可以将子查询移动到having子句。正如戈登回答的那样，你使用having子句作为第二个where，只有MySQL支持。最好使用where向and添加第二个条件：

SELECT e.id
FROM events e
WHERE author_id = 32
    AND e.id >= (SELECT MIN(u.id) id
        FROM (SELECT MIN(id) id
            FROM events
            WHERE author_id = 32
            GROUP BY type, post_id, table_code, comment_id, context
            ORDER BY MIN(id) desc 
            LIMIT 15) as u
        ) 
ORDER BY id DESC

根据您的评论，这会更简单一些。它选择具有最高事件ID的15个帖子：

SELECT  id
FROM    events
WHERE   author_id = 32
        AND post_id IN
        (
        SELECT  DISTINCT post_id
        FROM    events
        ORDER BY
                id DESC
        LIMIT   15
        )

Answer 2

你不能，但你可以将逻辑移到FROM条款：

SELECT e.id
FROM events e JOIN
     (SELECT MIN(u.id) as id
      FROM (SELECT MIN(id) as id
            FROM events
            WHERE author_id = 32
            GROUP BY type, post_id, table_code, comment_id, context
            ORDER BY MIN(id) desc 
            LIMIT 15
           ) u
     ) e15
     ON e.id >= e15.id
WHERE e.author_id = 32
ORDER BY e.id DESC;

我更喜欢这种配方，因为它是标准的SQL。您对HAVING子句的使用是MySQL扩展。

Answer 3

您还可以声明一个表并在其中插入指标ID记录，并在查询中使用它。我知道这太长了，但还有一种方法可以做到。

 Declare @indicatoridtable table
(
  @indicatorid datatype 
)

Insert into @indicatoridtable
SELECT MIN(u.id) as id
  FROM (SELECT MIN(id) as id
        FROM events
        WHERE author_id = 32
        GROUP BY type, post_id, table_code, comment_id, context
        ORDER BY MIN(id) desc 
        LIMIT 15
       ) 


 SELECT e.id
 FROM events e 
 ON e.id >= (select top 1 @indicatorid from @indicatoridtable )
 WHERE e.author_id = 32
 ORDER BY e.id DESC;

在将其用于HAVING子句时，如何省略SELECT语句中的列？

3 个答案: