Using SELECT

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

Using SET

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

Then you can use SELECT to show the value of @ModelID or use the variable into your code.

SELECT @ModelID

See this question for the difference between using SELECT and SET in TSQL.

Warning

If this SELECT statement returns multiple values (bad to begin with):

• When using SELECT, the variable is assigned the last value that is returned (as womp said), without any error or warning (this may cause logic bugs)
• The SET query returns error only if you DON'T put the semicolon in the end of query

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

If your select statement returns multiple values, your variable is assigned the last value that is returned.

For reference on using SELECT with variables: http://msdn.microsoft.com/en-us/library/aa259186%28SQL.80%29.aspx

declare @ModelID uniqueidentifer

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

select @ModelID

I prefer just setting it from the declare statement

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

Use TOP 1 if the query returns multiple rows.

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

There are three approaches:

1. DECLARE
2. SET -- Microsoft Recommended approach
3. SELECT

Below query details the advantage and disadvantage of each:

-- 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

You can use this, but remember that your query gives 1 result, multiple results will throw the exception.

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

Another way:

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

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

In this case if you have two or more results returned then your result is the last record. So be aware of this if you might have two more records returned as you might not see the expected result.

To ASSIGN variables using a SQL select the best practice is as shown below

->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;

IF you have to assign more than one variable in a single line you can use this same SELECT INTO

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

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

--HAPPY CODING-- 

My use case was that I wanted to set a variable to a string. All the other answers here show how to set a variable using the output of a SELECT statement. Here's how to do it with a simple string:

DECLARE @ModelID varchar(60) = 'Model T'