Re: can not use the column after rename

Hello Simon,

This question was raised up during writing a query where many columns are a result of long mathematical operations, so I thought why not to rename them and use the new names  instead of repeating the same operations

On the row level,  the value of a and b are know, so why we need a temporary table or CTE. Why I can not use it directly  as shown in the example below. why a and b can not be used as aliases for the column names ? I want to know the theoretical reason behind it?

Thanks


CREATE TABLE numbers (
    num1 integer,
    num2 integer
 );

 INSERT INTO numbers values (3,4);

 SELECT num1, num2, num1 + num2 FROM numbers

  SELECT num1 as a, num2 as b, a + b as c FROM number
  SELECT num1 as a, num2 as b, num1 + num2 as c FROM numbers

This is equivalent:

WITH numbers AS (SELECT 1 AS a, 2 AS b) SELECT a, b, a+b AS c FROM numbers;

S