如何獲取SQL函數運行不同的查詢並從任一查詢返回值？

Question

我需要的功能，但似乎無法得到它完全正確，我已經看過這裏和其他地方的例子，似乎無法得到這個剛剛好，我需要一個可選的項目列入我的查詢中，我有這個疑問（它的工作原理）：

SELECT TOP 100 PERCENT SKU, Description, LEN(CONVERT(VARCHAR 
(1000),Description)) AS LenDesc FROM tblItem 
WHERE Title = @Title AND Manufacturer = @Manufacturer 
ORDER BY LenDesc DESC 

這個工作在函數中，但是製造商是可選本次搜索的 - 這是要找到一個類似項目的描述，如果沒有出現，其它的查詢是：

SELECT TOP 100 PERCENT SKU, Description, LEN(CONVERT(VARCHAR 
(1000),Description)) AS LenDesc FROM tblItem 
WHERE Title = @Title ORDER BY LenDesc DESC 

一個缺少製造商，如何讓我的功能，使用基於製造商V無論是查詢存在或不存在。 原因是我有一個功能，首先檢查一個SKU的描述，如果它不存在 - 它使用此方法從類似產品獲取描述，然後使用相似產品的描述更新正在添加的產品。

這是迄今爲止功能：

ALTER FUNCTION [dbo].[GetDescriptionByTitleManufacturer] 
( 
    @Title varchar(400), 
    @Manufacturer varchar(160) 
) 
RETURNS TABLE 
AS 
RETURN (
    SELECT TOP 100 PERCENT SKU, Description, LEN(CONVERT(VARCHAR 
    (1000),Description)) AS LenDesc FROM tblItem 
    WHERE Title = @Title AND Manufacturer = @Manufacturer 
    ORDER BY LenDesc DESC 
    ) 

我已經嘗試添加開始和IF ...別人的，但得到的錯誤或語法問題，每一個方式，我試試吧，我希望能夠做一些事情像這樣的僞功能（不工作）：

ALTER FUNCTION [dbo].[GetDescriptionByTitleManufacturer] 
( 
    @Title varchar(400), 
    @Manufacturer varchar(160) 
) 
RETURNS TABLE 
AS 
BEGIN 
IF (@Manufacturer = Null) 
RETURN (
    SELECT TOP 100 PERCENT SKU, Description, LEN(CONVERT(VARCHAR 
    (1000),Description)) AS LenDesc FROM tblItem 
    WHERE Title = @Title ORDER BY LenDesc DESC 
    ) 
ELSE 
RETURN (
    SELECT TOP 100 PERCENT SKU, Description, LEN(CONVERT(VARCHAR 
    (1000),Description)) AS LenDesc FROM tblItem 
    WHERE Title = @Title AND Manufacturer = @Manufacturer 
    ORDER BY LenDesc DESC 
    ) 

END

Answer 1

你能不能做這樣的事？

ALTER FUNCTION [dbo].[GetDescriptionByTitleManufacturer] 
( 
    @Title varchar(400), 
    @Manufacturer varchar(160) 
) 
RETURNS TABLE 
AS 
RETURN (
    SELECT TOP 100 PERCENT SKU, Description, LEN(CONVERT(VARCHAR 
    (1000),Description)) AS LenDesc FROM tblItem 
    WHERE Title = @Title AND (@Manufacturer IS NULL OR Manufacturer = @Manufacturer) 
    ORDER BY LenDesc DESC 
    ) 

Answer 2

你可以嘗試使用NULL製造商犯規申請時：

SELECT TOP 100 PERCENT SKU, Description, LEN(CONVERT(VARCHAR 
(1000),Description)) AS LenDesc FROM tblItem 
WHERE Title = @Title AND (Manufacturer = @Manufacturer OR @Manufacturer IS NULL) 
ORDER BY LenDesc DESC 

Answer 3

當你做你AND (Manufacturer = @Manufacturer OR @Manufacturer IS NULL)消除使用索引。動態搜索有許多微妙的性能影響，您應該閱讀Dynamic Search Conditions in T-SQL by Erland Sommarskog

如果您擁有正確的SQL Server 2008版本（SQL 2008 SP1 CU5（10.0.2746）及更高版本），則可以使用此小技巧實際使用索引：

添加OPTION (RECOMPILE)到您的查詢，see Erland's article，和SQL Server將解決從(@Manufacturer IS NULL OR Manufacturer = @Manufacturer)內OR查詢計劃之前創建基於本地變量的值，可以使用索引。

ALTER FUNCTION [dbo].[GetDescriptionByTitleManufacturer] 
( 
    @Title varchar(400), 
    @Manufacturer varchar(160) 
) 
RETURNS TABLE 
AS 
RETURN 
(SELECT TOP 100 
    PERCENT SKU, Description, LEN(CONVERT(VARCHAR(1000),Description)) AS LenDesc 
    FROM tblItem 
     WHERE Title = @Title 
     AND (@Manufacturer IS NULL OR Manufacturer = @Manufacturer) 
    ORDER BY LenDesc DESC 
    OPTION (RECOMPILE) ---<<<<only valid for SQL 2008 SP1 CU5 (10.0.2746) and later 
) 
GO