Generate serial number in mysql query

Best Answer to this question should be the following, the best practice

SET @count:=0;
SELECT  (@count:=@count+1) AS serial_number, 
        marks 
FROM    student_marks

Based on your reasons for not wanting to use user defined variables as wanting to avoid having 2 queries, one for inializing and one to use it you could use the following:

SELECT  @a:=@a+1 serial_number, 
        marks 
FROM    student_marks,
        (SELECT @a:= 0) AS a;