Question

我有一个Sql Server查询：

SELECT  AC.Author_ID, A.Author_Name, AC.Paper_ID, AC.CoAuthor_ID, AP.Venue_ID, AC.Year
FROM    AuthorCoAuthor  AC
JOIN    AuthorPaper     AP ON AP.Author_ID  = AC.Author_ID AND 
                        AP.Paper_ID = AC.Paper_ID
JOIN    Author          A  ON A.Author_ID   = AC.Author_ID
ORDER BY AC.Author_ID, AC.Year, AC.Paper_ID, AC.CoAuthor_ID, AP.Venue_ID

返回近221317行。示例输出为：

Author_ID | Author_Name | Paper_ID | CoAuthor_ID | Venue_ID | Year  
------------------------------------------------------------------
677       | Nuno Vasc   | 812229   | 901706      | 64309    | 2005
677       | Nuno Vasc   | 812486   | 901706      | 65182    | 2005
677       | Nuno Vasc   | 818273   | 901706      | 185787   | 2007  
1359      | Peng Sui    | 818373   | 457348      | 18534    | 2005  
1359      | Peng Sui    | 868273   | 542321      | 184222   | 2006  
...       | ...         | ...      | ...         | ...      | ...

我希望查询只返回10个不同的Author_ID，而我尝试添加

WHERE  COUNT(DISTINCT(AC.Author_ID)) <= 10

这个WHERE子句进入同一个查询，但我仍然得到相同数量的行，即通常查询返回的所有数据。

为什么我的WHERE条款在这里不起作用？

Answer 1

要获得10个不同的Author_ID，您可以将 GROUP BY 与 TOP 10 一起使用。然后将结果（此处为10行）与其他表一起在select list中显示所需的列。

你可以试试这个：

SELECT AC.Author_ID, A.Author_Name, AC.Paper_ID, AC.CoAuthor_ID, AP.Venue_ID, AC.Year
FROM
(
   SELECT TOP (10) AC.Author_ID
   FROM AuthorCoAuthor AC
   GROUP BY AC.Author_ID
   ORDER BY AC.Author_ID
) DS
JOIN AuthorCoAuthor  AC ON AC.Author_ID = DS.Author_ID
JOIN    AuthorPaper     AP ON AP.Author_ID  = AC.Author_ID AND 
                    AP.Paper_ID = AC.Paper_ID
JOIN    Author          A  ON A.Author_ID   = AC.Author_ID
ORDER BY AC.Author_ID, AC.Year, AC.Paper_ID, AC.CoAuthor_ID, AP.Venue_ID

<强>更新

根据之前的答案，如果10 Distinct Author和AuthorPaper和Author同时，您可以获得少于10 Distinct Author的答案。 10 Distinct Author表没有其中一个十大连续作者。出于这个原因，您需要在获得AuthorCoAuthor时检查这些情况，以便当{且仅10 Distinct Author至少10 Distinct Author时，它始终为您提供SELECT AC.Author_ID, A.Author_Name, AC.Paper_ID, AC.CoAuthor_ID, AP.Venue_ID, AC.Year FROM ( SELECT TOP (10) AC.Author_ID FROM AuthorCoAuthor AC JOIN AuthorPaper AP ON AP.Author_ID = AC.Author_ID AND AP.Paper_ID = AC.Paper_ID JOIN Author A ON A.Author_ID = AC.Author_ID GROUP BY AC.Author_ID ORDER BY AC.Author_ID ) DS JOIN AuthorCoAuthor AC ON AC.Author_ID = DS.Author_ID JOIN AuthorPaper AP ON AP.Author_ID = AC.Author_ID AND AP.Paper_ID = AC.Paper_ID JOIN Author A ON A.Author_ID = AC.Author_ID ORDER BY AC.Author_ID, AC.Year, AC.Paper_ID, AC.CoAuthor_ID, AP.Venue_ID。所以，为了确保你总是得到log，你可以使用这个：

env_logger

从Sql Server列中选择编号的Distinct值

1 个答案: