declare @ModelID uniqueidentifer

--make sure to use brackets
set @ModelID = (select modelid from models
where areaid = 'South Coast')

select @ModelID

declare @ModelID uniqueidentifer

--make sure to use brackets
set @ModelID = (select modelid from models
where areaid = 'South Coast')

select @ModelID

有三种方法:

声明 SET——微软推荐的方法 选择

下面的查询详细说明了每种方法的优点和缺点:

-- First way, 
DECLARE @test int = (SELECT 1)
       , @test2 int = (SELECT a from (values (1),(2)) t(a)) -- throws error

-- advantage: declare and set in the same place
-- Disadvantage: can be used only during declaration. cannot be used later

-- Second way
DECLARE @test int  
       , @test2 int 

SET @test = (select 1)
SET @test2 = (SELECT a from (values (1),(2)) t(a)) -- throws error

-- Advantage: ANSI standard. 
-- Disadvantage: cannot set more than one variable at a time

-- Third way
DECLARE @test int, @test2 int 
SELECT @test = (select 1)
      ,@test2 = (SELECT a from (values (1),(2)) t(a)) -- throws error

-- Advantage: Can set more than one variable at a time
-- Disadvantage: Not ANSI standard

我更喜欢从declare语句中设置它

DECLARE @ModelID uniqueidentifer = (SELECT modelid 
                                    FROM models
                                    WHERE areaid = 'South Coast')

Select @ModelID =m.modelid 
From   MODELS m
Where  m.areaid = 'South Coast'

在这种情况下，如果你有两个或更多的结果返回，那么你的结果是最后的记录。因此，如果您可能会返回另外两条记录，因为您可能看不到预期的结果，请注意这一点。

如果查询返回多行，则使用TOP 1。

SELECT TOP 1 @ModelID = m.modelid 
  FROM MODELS m
 WHERE m.areaid = 'South Coast'