如何在SQl Server 2008中选择distinct,但仅限于多个中的一个字段？

Question

我有一个问题:

SELECT Content.content_name, Language2.Name, Language2.language_id, 
Content.id, Content.content_description, 
FROM Language AS Language2 
LEFT JOIN contents AS Content ON (Language2.language_id = Content.language_id) 

如何仅选择不同的content_name？

Answer 1

你做这个:

SELECT DISTINCT Content.content_name
FROM Language AS Language2 
LEFT JOIN contents AS Content ON (Language2.language_id = Content.language_id)

那么为什么这不能回答你的问题呢？

让我们考虑以下数据(只是前两列):

content_name      Name
 XXXXX            1234
 XXXXX            5678

SELECT DISTINCT 意味着你只想要一行,但你想要什么名字？

您需要做的是重写要使用的代码GROUP BY并为其他列选择适当的聚合函数:

SELECT
    Content.content_name,
    MIN(Language2.Name) AS Name,
    MIN(Language2.language_id) AS language_id, 
    MIN(Content.id) AS id,
    MIN(Content.content_description) AS content_description, 
FROM
    Language AS Language2 
    LEFT JOIN contents AS Content
        ON (Language2.language_id = Content.language_id) 
GROUP BY
    Content.content_name

现在,这可能不会产生你想要的东西,但有一件事是肯定的,你不能欺骗数据库引擎只是"选择其中一行返回,我不关心哪一行."

Answer 2

WITH    q AS
        (
        SELECT  Content.content_name, Language2.Name, Language2.language_id, Content.id, Content.content_description, ROW_NUMBER() OVER (PARTITION BY content_name ORDER BY language_id) AS rn
        FROM    Language Language2
        LEFT JOIN
                Contents AS Content
        ON      Language2.language_id = Content.language_id
        )
SELECT  *
FROM    q
WHERE   rn = 1