在一个列上选择DISTINCT，同时返回其他列？

我有一个查询，该查询使用三个查找表来获取我需要的所有信息。我需要DISTINCT为一列提供值，但是我还需要与之关联的其余数据。

我的SQL代码：

SELECT acss_lookup.ID AS acss_lookupID,
   acss_lookup.product_lookupID AS acssproduct_lookupID,
   acss_lookup.region_lookupID AS acssregion_lookupID,
   acss_lookup.document_lookupID AS acssdocument_lookupID,
   product.ID AS product_ID,
   product.parent_productID AS productparent_product_ID,
   product.label AS product_label,
   product.displayheading AS product_displayheading,
   product.displayorder AS product_displayorder,
   product.display AS product_display,
   product.ignorenewupdate AS product_ignorenewupdate,
   product.directlink AS product_directlink,
   product.directlinkURL AS product_directlinkURL,
   product.shortdescription AS product_shortdescription,
   product.logo AS product_logo,
   product.thumbnail AS product_thumbnail,
   product.content AS product_content,
   product.pdf AS product_pdf,
   product.language_lookupID AS product_language_lookupID,
   document.ID AS document_ID,
   document.shortdescription AS document_shortdescription,
   document.language_lookupID AS document_language_lookupID,
   document.document_note AS document_document_note,
   document.displayheading AS document_displayheading
FROM acss_lookup
     INNER JOIN product ON (acss_lookup.product_lookupID = product.ID)
     INNER JOIN document ON (acss_lookup.document_lookupID = document.ID)
ORDER BY product_displayheading ASC;

我想从该查询中获取所有产品，但我只想获取一次，因为我正在填充搜索应用程序的下拉菜单。我希望用户能够从该表中的产品中进行选择（这就是为什么我只需要它们一次的原因）。

这太复杂了吗？我应该使用更简化的方法吗？

尚未提及的另一种方法是使用窗口函数，例如row_number：

   SELECT * FROM  
   (
   SELECT acss_lookup.ID AS acss_lookupID, 
   ROW_NUMBER() OVER 
   (PARTITION BY your_distinct_column ORDER BY any_column_you_think_is_appropriate)
   as num,
   acss_lookup.product_lookupID AS acssproduct_lookupID,
   acss_lookup.region_lookupID AS acssregion_lookupID,
   acss_lookup.document_lookupID AS acssdocument_lookupID,
   product.ID AS product_ID,
   product.parent_productID AS productparent_product_ID,
   product.label AS product_label,
   product.displayheading AS product_displayheading,
   product.displayorder AS product_displayorder,
   product.display AS product_display,
   product.ignorenewupdate AS product_ignorenewupdate,
   product.directlink AS product_directlink,
   product.directlinkURL AS product_directlinkURL,
   product.shortdescription AS product_shortdescription,
   product.logo AS product_logo,
   product.thumbnail AS product_thumbnail,
   product.content AS product_content,
   product.pdf AS product_pdf,
   product.language_lookupID AS product_language_lookupID,
   document.ID AS document_ID,
   document.shortdescription AS document_shortdescription,
   document.language_lookupID AS document_language_lookupID,
   document.document_note AS document_document_note,
   document.displayheading AS document_displayheading
   FROM acss_lookup
     INNER JOIN product ON (acss_lookup.product_lookupID = product.ID)
     INNER JOIN document ON (acss_lookup.document_lookupID = document.ID)
   )a
   WHERE a.num = 1
   ORDER BY product_displayheading ASC;

有几种方法可以做到这一点。我使用的两个主要参数是公用表表达式和子查询。使用CTE，您的查询将如下所示：

WITH theResultSet AS
(
    SELECT DISTINCT(column) AS col1 FROM some.table
)
SELECT whatever
  FROM more.data AS a
  JOIN theResultSet as b ON a.col1 = b.col1
  /* additional joins, clauses etc...*/

或使用子查询：

SELECT whatever
  FROM more.data AS a
  JOIN (SELECT DISTINCT(column) AS col1 FROM some.table) AS b ON a.col1 = b.col1
/* additional joins, clauses etc... */

我通常会测试一下哪个更快，然后选择那个。

我希望这可以帮助你。

^{（我认为您想要做的是将每个结果行“折叠”为一个产品，因此，此答案就是基于这种假设。）}

这是不可能的。要从其他表中获取相关的1 .. *数据，您必须在其他列中返回重复的值。

通常，处理此问题的方法是按原样运行查询，并处理应用程序代码中的合并结果集。我通常使用哈希收集方法来完成此操作，该方法以基于键值的集合中每种类型的不同实体结束。

尽管此方法确实会花费更多的网络流量，但通常最好执行一些操作，例如运行多个查询并将结果按需要在应用程序代码中组合在一起。它取决于许多因素，包括查询/查询运行的频率和返回多少数据。