TSQL如何在子查詢中重用記錄

Question

我有一個類似於下面的查詢，我需要從另一個表中返回幾個字段作爲子查詢。有沒有一種方法我只能將查詢調用到TABLE2一次，並使用該查詢的結果來填充我需要的各個字段？我正在尋找類似於如何在LINQ中使用let語句的東西。謝謝。

select 

(select field1 from TABLE2 where id = 1) as field1, 
(select field2 from TABLE2 where id = 1) as field2, 
(select field3 from TABLE2 where id = 1) as field3, 
(select field4 from TABLE2 where id = 1) as field4 

from mainTable where p1 = @p1 

Answer 1

我會用OUTER APPLY：

select 

x.field1 as field1, 
x.field2 as field2, 
x.field3 as field3, 
x.field4 as field4 

from mainTable as mt 
outer apply (select field1, field2, field3, field4 from TABLE2 where id = 1) as x 
where p1 = @p1 

Answer 2

您可以使用CROSS JOIN：

select t.field1, t.field2, ... 
from mainTable 
cross join (select field1, field2, ... from TABLE2 where id = 1) as t 
where p1 = @p1 

我認爲在cross join操作中使用的子查詢返回一個記錄。

Answer 3

我認爲變量是最接近LINQ 讓條款。例如：

declare @field1 bigint; 
declare @field2 nvarchar(64); 

select @field1 = field1, @field2=field2 from TABLE2 where id = 1; 


select @field1, @field2, SomeOtherField from mainTable 
where p1 = @p1; 

當然，您需要設置適當類型的變量。