Query to split string from comma

Question

DECLARE @str varchar(50)='Hello, world'; /* SQL SERVER QUERY TO SPLIT STRING FROM COMMA AND GET LEFT AND RIGHT PART */

SELECT LEFT(@str, CHARINDEX(',', @str)-1) as LeftPart, RIGHT(@str, LEN(@str)-CHARINDEX(',', @str)) as RightPart;

Error is

Must declare the scalar variable "@str"

 

why it is coming ? if any one know tell me friends