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
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;