如何从SQL查询中设置变量？

Question

我正在尝试从SQL查询中设置变量:

declare @ModelID uniqueidentifer

Select @ModelID = select modelid from models
where areaid = 'South Coast'

显然我没有这样做,因为它不起作用.有人可以提出解决方案吗？

谢谢!

Answer 1

使用SELECT:

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

使用SET:

SET @ModelID = (SELECT m.modelid 
                  FROM MODELS m
                 WHERE m.areaid = 'South Coast')

有关在TSQL中使用SELECT和SET之间的区别,请参阅此问题.

警告

如果此select语句返回多个值(开头不好):

• 使用时SELECT,变量被赋予最后返回的值(如womp所述),没有任何错误或警告(这可能导致逻辑错误)
• 使用时SET,将发生错误

Answer 2

SELECT @ModelID = modelid
FROM Models
WHERE areaid = 'South Coast'

如果您的select语句返回多个值,则会为您的变量分配返回的最后一个值.

有关对变量使用SELECT的参考:http://msdn.microsoft.com/en-us/library/aa259186%28SQL.80%29.aspx

Answer 3

declare @ModelID uniqueidentifer

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

select @ModelID

Answer 4

我更喜欢从declare语句中设置它

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

Answer 5

TOP 1如果查询返回多行,请使用.

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

Answer 6

有以下三种方法：

1. 宣布
2. SET -- 微软推荐的方法
3. 选择

下面的查询详细说明了每种方法的优缺点：

-- 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

Answer 7

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

在这种情况下，如果返回两个或多个结果，则您的结果是最后一条记录。因此，如果您可能会返回另外两条记录，请注意这一点，因为您可能看不到预期结果。

Answer 8

要使用 SQL 选择分配变量，最佳实践如下所示

->DECLARE co_id INT ;
->DECLARE sname VARCHAR(10) ;

->SELECT course_id INTO co_id FROM course_details ;
->SELECT student_name INTO sname FROM course_details;

如果您必须在一行中分配多个变量，您可以使用相同的 SELECT INTO

->DECLARE val1 int;
->DECLARE val2 int;

->SELECT student__id,student_name INTO val1,val2 FROM student_details;

--HAPPY CODING-- 

Answer 9

您可以使用它，但是请记住您的查询给出1个结果，多个结果将引发异常。

declare @ModelID uniqueidentifer
Set @ModelID = (select Top(1) modelid from models where areaid = 'South Coast')

其它的办法：

Select Top(1)@ModelID = modelid from models where areaid = 'South Coast'