TSQL如何在子查询中重用记录

Question

我有一个类似于下面的查询，我需要从另一个表中返回几个字段作为子查询。有没有一种方法我只能将查询调用到TABLE2一次，并使用该查询的结果来填充我需要的各个字段？我正在寻找类似于如何在LINQ中使用let语句的东西。谢谢。

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

我会用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 

我认为在cross join操作中使用的子查询返回一个记录。

Answer 3

我认为变量是最接近LINQ 让条款。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; 

当然，您需要设置适当类型的变量。