What is the difference between SET and SELECT when assigning values to variables, in T-SQL?

http://vyaskn.tripod.com/differences_between_set_and_select.htm

 

https://stackoverflow.com/questions/3945361/set-versus-select-when-assigning-variables

 

• SET is the ANSI standard for variable assignment, SELECT is not.

 

• SET can only assign one variable at a time, SELECT can make multiple assignments at once.

 

• If assigning from a query, SET can only assign a scalar value. If the query returns multiple values/rows then SET will raise an error. SELECT will assign one of the values to the variable and hide the fact that multiple values were returned (so you'd likely never know why something was going wrong elsewhere - have fun troubleshooting that one)

 

• When assigning from a query if there is no value returned then SET will assign NULL, where SELECT will not make the assignment at all (so the variable will not be changed from its previous value)

 

• As far as speed differences - there are no direct differences between SET and SELECT. However SELECT's ability to make multiple assignments in one shot does give it a slight speed advantage over SET.

 

http://www.sql-server-helper.com/tips/set-vs-select-assigning-variables.aspx

SET	SELECT
ANSI standard for variable assignment.	Non-ANSI standard when assigning variables.
Can only assign one variable at a time. SET @Index = 1 SET @LoopCount = 10 SET @InitialValue = 5	Can assign values to more than one variable at a time. SELECT @Index = 1, @LoopCount = 10, @InitialValue = 5
When assigning from a query and the query returns no result, SET will assign a NULL value to the variable. DECLARE @CustomerID NCHAR(5) SET @CustomerID = 'XYZ' SET @CustomerID = (SELECT [CustomerID] FROM [dbo].[Customers] WHERE [CustomerID] = 'ABC') SELECT @CustomerID -– Returns NULL	When assigning from a query and the query returns no result, SELECT will not make the assignment and therefore not change the value of the variable. DECLARE @CustomerID NCHAR(5) SET @CustomerID = 'XYZ' SELECT @CustomerID = [CustomerID] FROM [dbo].[Customers] WHERE [CustomerID] = 'ABC' SELECT @CustomerID –- Returns XYZ
When assigning from a query that returns more than one value, SET will fail with an error. SET = (SELECT [CustomerID] FROM [dbo].[Customers]) Msg 512, Level 16, State 1, Line 3 Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.	When assigning from a query that returns more than one value, SELECT will assign the last value returned by the query and hide the fact that the query returned more than one row. SELECT @CustomerID = [CustomerID] FROM [dbo].[Customers] -- No error generated

作者:Chuck Lu    GitHub