LINQ在T-SQL中推遲執行的等效代碼

Question

在LINQ中，可以利用延遲執行在設計時分離彼此的不同關注點。我的提供程序將生成一個最佳的SQL語句來獲取數據。請看下面的例子：

// some complex query to fetch matches goes here 
IEnumerable<Match> data = null; 

if (/* some condition */) 
{ 
    data = from m in Context.Matches 
     where /* ... */ 
     select m; 
} 
else 
{ 
    data = from m in Context.Matches 
     join /* .. */ 
     join /* .. */ 
     where /* ... */ 
     select m; 
} 

// later on, order the data depending on some user selection 
IOrderedEnumerable<Match> orderedData = null; 

switch (someOption) 
{ 
    case /*...*/: 
     orderedData = from m in data orderby by m.Start select m; 
     break; 
    case /*...*/ 
     orderedData = from m in data orderby by m.ID select m; 
     break; 
} 

// do something with the computed data 

是否有一個有效的方式來實現在存儲過程中相同的功能，而不需要構建查詢作爲一個字符串？我所能想到的只是表變量，但我不知道它們對性能的影響：

CREATE @matches TABLE 
(
    ID int, 
    -- ... 
    Start datetime 
) 


-- some complex query to fetch matches goes here 
IF -- some condition 
    INSERT INTO @matches(ID, Start) 
     SELECT ID, ..., Start FROM Matches WHERE ... 
ELSE 
    INSERT INTO @matches(ID, Start) 
     SELECT ID, ..., Start FROM Matches JOIN ... JOIN ... WHERE ... 

-- later on, order the data depending on some user selection 
CREATE @orderedMatches TABLE 
(
    ID int, 
    -- ... 
    Start datetime 
) 

IF -- some option 
    INSERT INTO @orderedMatches (ID, Start) 
     SELECT ID, ..., Start FROM @matches ORDER BY Start ASC 
ELSE IF -- some option 
    INSERT INTO @orderedMatches (ID, Start) 
     SELECT ID, ..., Start FROM @matches ORDER BY ID ASC 

-- do something with the computed data 

Answer 1

延遲執行在T-SQL中不存在。您使用的任何聲明都將立即執行。你可以得到的最接近的東西是動態SQL：構建一個包含你的邏輯SQL文本，從片，最後執行SQL：

declare @sql nvarchar(max) = N''; 

if <some condition> 
@sql += N'SELECT ...' 

if <some other condition> 
@sql += N'INSERT ...' 

-- finally, execute the SQL: 
exec sp_executesql @sql; 

不用說，這是遠不及強大的LINQ。做任何事情像添加可選的WHERE子句都需要複雜的邏輯來構建@sql腳本。這也會導致代碼出錯並且極難調試。閱讀The Curse and Blessings of Dynamic SQL進行更徹底的討論。

SQLCLR是一種替代方案，如果你願意走這條路。