Is it better to populate variables using SET or SELECT?

Question

What is the better way (with regards to performance) to set a value to variable?

1. By SET command:

   DECLARE @VarString nvarchar(max);
   SET @VarString = 'john doe';
   SELECT @VarString;
   

2. By SELECT command:

   DECLARE @VarString nvarchar(max);
   SELECT @VarString = 'john doe';
   SELECT @VarString;
   

Answer 1

Don't pick one or the other strictly for performance.

Pick based on what fits your style and development needs, as is advised in the conclusion to this excellent comparison between SELECT and SET (emphasis and minor formatting added):

Best practice suggests not to stick to one method. Depending on the scenario you may want to use both SET or SELECT.

Following are few scenarios for using SET:

• If you are required to assign a single value directly to variable and no query is involved to fetch value
• NULL assignments are expected (NULL returned in result set)
• Standards are meant to be follow for any planned migration
• Non scalar results are expected and are required to be handled

Using SELECT is efficient and flexible in the following few cases:

• Multiple variables are being populated by assigning values directly
• Multiple variables are being populated by single source (table , view)
• Less coding for assigning multiple variables
• Use this if you need to get @@ROWCOUNT and @ERROR for last statement executed

Answer 2

I think the consensus is there's a negligible performance difference between the two (with SET being faster). But if you are like me, I'd use SET where appropriate for readability purposes. The next guy who will maintain your code will thank you for that ;-)

Answer 3

I always use SET unless I am populating multiple variables from the result set of 1 query. That way, by using SELECT we only query the table once.