reusing alias in SELECT

Question

What I am trying to do is add another column that calculates (cr - dr)

Seeing as you cannot re-use an alias inside a SELECT clause, how would you go about calculatin total

    SELECT SUM(b.bet_win * cy.fx_rate )as dr, SUM(b.bet_loss * cy.fx_rate ) as cr, cr+dr as total
    FROM ....
    WHERE ....

Answer 1

In SQL Server or Oracle, I'd use a CTE, but since you're using MySQL, you'd use a subquery:

SELECT dr, cr, cr + dr as total 
FROM (
    SELECT 
         SUM(b.bet_win * cy.fx_rate ) as dr, 
         SUM(b.bet_loss * cy.fx_rate ) as cr
    FROM ....
    WHERE ....) t;