how to declare a variable in sql

1-- Declare the variable to be used.
2DECLARE @MyCounter int;
3
4-- Initialize the variable.
5SET @MyCounter = 0;

1DECLARE @TABLE TABLE( COL1 INT, COL2 VARCHAR(30))

1DECLARE @COURSE_ID AS INT, @COURSE_NAME VARCHAR (10);
2

1DECLARE  { @LOCAL_VARIABLE[AS] data_type  [ = value ] } 

1One way of doing this is to use database variables
2
3create table testcalc (
4   id int(11) not null auto_increment,
5   num1 int(11) default null,
6   num2 int(11) default null,
7   num3 int(11) default null,
8   num4 int(11) default null,
9   primary key(id)
10);
11
12insert into testcalc values
13(default, 1, 2, 3, 4),
14(default, 5, 10, 15, 20);
15Then you can get the same results as in your example by storing the calculation results in variable syntax like this
16
17@youVar := (calc) as resultName01
18Then it will be available to following calculations to use like this
19
20(@youVar + newCalc) as resultName02
21We can apply it to your example like this
22
23select
24   id,
25   num1,
26   num2,
27   num3,
28   num4,
29   @1plus2 := (num1 + num2) as 1plus2,                   # create var01
30   @1plus2mult3 := (@1plus2 * num3) as 1plus2mult3,      # create var02 using var01
31   @sumOfCalc := (@1plus2 + @1plus2mult3) as sumOfCalc   # create var03 using var01 and var02
32from testcalc;