TSQL - IN子句返回空時的替代結果集

Question

我需要幫助才能在更大的CTE查詢中實現篩選器。

想象一下這樣的情景：可能屬於幾個類別的項目

• 名單。
• 用戶遵循這些類別之一。
• 過濾器：用戶可以指定當一個類別被遵循時，他/她只想查看也屬於他/她可以指定的至少一個輔助類別的項目。

示例：用戶遵循「計算機」類別並指定僅希望查看「平板電腦」。當用戶查看「計算機」時，查詢應該只選擇也屬於「平板電腦」類別的項目。如果沒有指定過濾器，則應顯示主類別（「計算機」）的所有項目。

SQL小提琴似乎在一瞬間有一些問題，so here's a little schema and query (at the end)這說明問題

SELECT * FROM ItemsToShowWithCategories itswh_cte1 
WHERE /* Other filters AND */ 
    itswh_cte1.ItemPk IN (
     SELECT DISTINCT itswh_cte2.ItemPk 
     FROM ItemsToShowWithCategories AS itswh_cte2 
     INNER JOIN ItemsToShowWithCategories AS itswh3_cte 
      ON itswh_cte2.ItemPk=itswh3_cte.ItemPk 
     LEFT JOIN CategoriesRequiredByCategory_CTE AS crbc_cte 
      ON itswh_cte2.FkCategory=crbc_cte.FkCategoryView 
      AND crbc_cte.FkCategoryRequired=itswh3_cte.FkCategory 
     WHERE crbc_cte.FkCategoryView IS NOT NULL 
    ); 

問：我怎麼能返回所有記錄時IN子句返回空？

Answer 1

好了，最明顯的將是這樣的：

WHERE /* Other filters AND */ 
    itswh_cte1.ItemPk IN (
     SELECT itswh_cte2.ItemPk 
     FROM ItemsToShowWithCategories AS itswh_cte2 
     INNER JOIN ItemsToShowWithCategories AS itswh3_cte 
      ON itswh_cte2.ItemPk=itswh3_cte.ItemPk 
     LEFT JOIN CategoriesRequiredByCategory_CTE AS crbc_cte 
      ON itswh_cte2.FkCategory=crbc_cte.FkCategoryView 
      AND crbc_cte.FkCategoryRequired=itswh3_cte.FkCategory 
     WHERE crbc_cte.FkCategoryView IS NOT NULL 
    ) 
    or 
    (SELECT count(ItemPk) 
     FROM ItemsToShowWithCategories AS itswh_cte2 
     INNER JOIN ItemsToShowWithCategories AS itswh3_cte 
      ON itswh_cte2.ItemPk=itswh3_cte.ItemPk 
     LEFT JOIN CategoriesRequiredByCategory_CTE AS crbc_cte 
      ON itswh_cte2.FkCategory=crbc_cte.FkCategoryView 
      AND crbc_cte.FkCategoryRequired=itswh3_cte.FkCategory 
     WHERE crbc_cte.FkCategoryView IS NOT NULL) = 0 

有幾個變種，你可以嘗試，如果這個表現不佳，例如使用(select top 1 ItemPk ...) is null或類似的東西。 CTE將允許您避免兩次編寫相同的SQL（如果這不是SQL生成的，則有用）。