How can I assign value to a variable using aggregate function in mysql?

Question

This is my table structure and data.

create table StudentInformation
(
    sId INT(5),
    name VARCHAR(50),
    sClass VARCHAR(10),
    maths INT(5),
    physics INT(5),
    chemistry INT(5)
);

INSERT INTO StudentInformation
values
(1, 'Jai', '11th', 60, 75, 65),
(2, 'Leela', '12th', 91, 87, 94),
(3, 'Suresh', '11th', 75, 68, 70),
(4, 'Ramesh', '11th', 50, 67, 55),
(5, 'Janki', '12th', 78, 89, 78),
(6, 'Lalu', '12th', 30, 38, 45),
(7, 'Amit', '11th', 91, 95, 93),
(8, 'Komal', '11th', 66, 78, 74),
(9, 'Sanjay', '12th', 25, 40, 35);

Now I want to calculate average marks for each class.
I have tried this query :

SELECT
sClass class,
@var := sum(maths+physics+chemistry)/(count(sid)*3) as avgMarksPerSubject,
@var as variableValue,
count(sid) as numberOfStudents
FROM StudentInformation
#where @var > 65
group by sClass;

Use of variable is compulsory here as this is just an example of my actual task.
Now I would like to have those records which have more than 65 marks.
Is it possible by using variable in WHERE clause ?
I am not getting actual data in @var , how can I use it in WHERE clause ?
You can try sql query here .
Any suggestion ?
Thanks

Answer 1

Using a user defined session variable in where clause is only possible when it is pre initialized . Unless otherwise, due to the SQL-Query-Order-of-Operations , the variable will be having a default NULL and the condition may not satisfy the results as expected.

set @var:=0;

SELECT
      sClass class,
      @var := cast(sum(maths+physics+chemistry)
                   /(count(sid)*3) as decimal(6,2)
              ) as avgMarksPerSubject,
      @var as variableValue,
      count(sid) as numberOfStudents
FROM  StudentInformation
where @var < 65
group by sClass
;

+-------+--------------------+---------------+------------------+
| CLASS | AVGMARKSPERSUBJECT | VARIABLEVALUE | NUMBEROFSTUDENTS |
+-------+--------------------+---------------+------------------+
| 11th  |              72.13 |             0 |                5 |
| 12th  |              60.83 |             0 |                4 |
+-------+--------------------+---------------+------------------+

Here you can clearly see that the variable is not assigned any value per row and from the value calculated in the previous column expression.

You can see its side effects by running the following query:

select * from (
  SELECT
      sClass class,
      @var := cast(sum(maths+physics+chemistry)
                   /(count(sid)*3) as decimal(6,2)
              ) as avgMarksPerSubject,
      @var as variableValue,
      count(sid) as numberOfStudents
  FROM StudentInformation
  group by sClass
) r where avgMarksPerSubject > 65

+-------+--------------------+---------------+------------------+
| CLASS | AVGMARKSPERSUBJECT | VARIABLEVALUE | NUMBEROFSTUDENTS |
+-------+--------------------+---------------+------------------+
| 11th  |              72.13 |         60.83 |                5 |
+-------+--------------------+---------------+------------------+

Example @ SQL Fiddle :