how to define a variable in sql code example

Example 1: sql declare variable

-- Declare the variable to be used.
DECLARE @MyCounter int;

-- Initialize the variable.
SET @MyCounter = 0;

Example 2: var in sql

One way of doing this is to use database variables

create table testcalc (
   id int(11) not null auto_increment,
   num1 int(11) default null,
   num2 int(11) default null,
   num3 int(11) default null,
   num4 int(11) default null,
   primary key(id)
);

insert into testcalc values
(default, 1, 2, 3, 4),
(default, 5, 10, 15, 20);
Then you can get the same results as in your example by storing the calculation results in variable syntax like this

@youVar := (calc) as resultName01
Then it will be available to following calculations to use like this

(@youVar + newCalc) as resultName02
We can apply it to your example like this

select
   id,
   num1,
   num2,
   num3,
   num4,
   @1plus2 := (num1 + num2) as 1plus2,                   # create var01
   @1plus2mult3 := (@1plus2 * num3) as 1plus2mult3,      # create var02 using var01
   @sumOfCalc := (@1plus2 + @1plus2mult3) as sumOfCalc   # create var03 using var01 and var02
from testcalc;