SQL Server：如何調用UDF（如果可用）？

Question

大多數系統將具有可用的用戶定義功能（UDF）。 有些不會。 如果要使用UDF，我想使用它：

SELECT 
    Users.*,
    dbo.UserGroupMembershipNames(Users.UserID) AS MemberOfGroupNames
FROM Users

否則退回到可接受的替代方案

SELECT
   Users.*,
   (SELECT TOP 1 thing FROM Something 
    WHERE Something.ID = Users.UserID) AS MemberGroupNames
FROM Users

怎么辦

---

我的第一次嘗試使用明顯的解決方案當然失敗了：

SELECT 
    Users.*,
    CASE
    WHEN (OBJECT_ID('dbo.UserGroupMembershipNames') IS NOT NULL) THEN
           dbo.UserGroupMembershipNames(Users.UserID)
    ELSE (SELECT TOP 1 thing FROM Something 
          WHERE Something.ID = Users.UserID)
    END AS MemberOfGroupNames
FROM Users

由於我以外的原因

Answer 1

您可能必須先檢查該函數的存在，然后再選擇是否調用它：

IF OBJECT_ID('dbo.UserGroupMembershipNames') IS NOT NULL
    SELECT 
        Users.*,
        dbo.UserGroupMembershipNames(Users.UserID) AS MemberOfGroupNames
    FROM Users
ELSE
    SELECT
        Users.*,
        (SELECT TOP 1 thing FROM Something 
         WHERE Something.ID = Users.UserID) AS MemberGroupNames
    FROM Users

我認為不可能將其捆綁在單個條件SELECT語句中。

Answer 2

這是因為引擎在執行計划的解析或准備期間嘗試綁定到UDF。

您將不得不使用條件語句將其拆分為兩個語句（很遺憾，這在視圖或內聯表值函數中不起作用）：

IF (OBJECT_ID('dbo.UserGroupMembershipNames') IS NOT NULL)
SELECT  
    Users.*, 
    dbo.UserGroupMembershipNames(Users.UserID) 
    AS MemberOfGroupNames 
FROM Users 
ELSE
SELECT  
    Users.*, 
    (SELECT TOP 1 thing FROM Something  
          WHERE Something.ID = Users.UserID) 
    AS MemberOfGroupNames 
FROM Users