消除 Join over 3 Tables 中的重复项

Question

我正在使用以下 select 语句在我的网站上填充已输入 3 个表的列表的网格。

BND_Listing = Main Data
BND_ListingCategories = My Category data for the Listing Table
BND_LingingJunction = Junction Table for multi-category listings.

我刚刚实现了 Junction 表，因为任何列表都可以有超过 1 个类别，但是当我有（使用 CategoryY 和 CategoryZ 的列表 X）时，我的问题出现了，因为 CatX 和 CatY 会在我的列表中显示两次在我的联结表上这个特定列表有 2 个条目，所以我理解为什么它会显示两次，但我希望所有内容都只显示一次，而不是每个类别显示一次。

SELECT DISTINCT * FROM BND_ListingJunction 
    JOIN BND_listing on BND_listing.LID = BND_ListingJunction.Junc_LID
    JOIN BND_ListingCategories 
       on BND_ListingCategories.CatID = 
            BND_ListingJunction.Junc_CatID
WHERE (CategoryName = '[querystring:filter-Category]' or
          '[querystring:filter-Category]'='All')
    and (City = '[querystring:filter-City]' or 
          '[querystring:filter-City]'='All')
    and (Region= '[querystring:filter-State]' or 
          '[querystring:filter-State]'='All')
    and (Country= '[querystring:filter-Country]' or 
          '[querystring:filter-Country]'='All')
    and isnull(Company,'') <> ''
ORDER by Company ASC

我意识到使用 distinct 在这里不起作用，因为它是从我的联结表中提取的第一个，也许查询需要以不同的方式组织？

Answer 1

因为 distinct 应用于查询的每个输出列。
City、Region、Country 和 Company 在哪个表中？如果他们在ListingJunction' orListingCategories`， 如果您需要不同的列表和类别列表，

SELECT DISTINCT l.*, c.CategoryName 
FROM BND_Listing l
   Join BND_ListingJunction j
      on j.Junc_LID = l.LID
   join BND_ListingCategories c
      on c.CatId = j.Junc_CatID
Where (c.CategoryName = '[querystring:filter-Category]' or
                    '[querystring:filter-Category]'='All')
      and (City = '[querystring:filter-City]' or 
                    '[querystring:filter-City]'='All')
      and (Region= '[querystring:filter-State]' or 
                    '[querystring:filter-State]'='All')
      and (Country= '[querystring:filter-Country]' or 
                    '[querystring:filter-Country]'='All')
      and isnull(Company,'') <> ''
Order by l.Company, l.LID 

如果你想要的只是不同的列表，试试这个：

SELECT DISTINCT * FROM BND_Listing l 
Where Exists
  (Select * from BND_ListingJunction j
      join BND_ListingCategories c
          on c.CatId = j.Junc_CatID
   Where j.Junc_LID = l.LID
      and (c.CategoryName = '[querystring:filter-Category]' or
                    '[querystring:filter-Category]'='All')
      and (City = '[querystring:filter-City]' or 
                    '[querystring:filter-City]'='All')
      and (Region= '[querystring:filter-State]' or 
                    '[querystring:filter-State]'='All')
      and (Country= '[querystring:filter-Country]' or 
                    '[querystring:filter-Country]'='All')
      and isnull(Company,'') <> '')
ORDER by l.Company ASC

如果它们在 `BND_Listing' 中，试试这个：

SELECT DISTINCT * FROM BND_Listing l 
Where Exists
      (Select * from BND_ListingJunction j
         join BND_ListingCategories c
             on c.CatId = j.Junc_CatID
       Where j.Junc_LID = l.LID
           and (c.CategoryName = '[querystring:filter-Category]' or
                    '[querystring:filter-Category]'='All'))
   and (l.City = '[querystring:filter-City]' or 
                    '[querystring:filter-City]'='All')
   and (l.Region= '[querystring:filter-State]' or 
                    '[querystring:filter-State]'='All')
   and (l.Country= '[querystring:filter-Country]' or 
                    '[querystring:filter-Country]'='All')
   and isnull(l.Company,'') <> ''
ORDER by l.Company ASC