How to use alias in SELECT - mysql

Question

I have the mysql statement below all works fine until I add (sub_total - discount_total) AS total,.
I'm guessing this is because I am calling on aliases that are not defined yet (sub_total and discount_total).
How do I define the aliases so it doesn't throw an error #1054 - Unknown column 'sub_total' in 'field list'
SELECT SQL_CALC_FOUND_ROWS
`bookings`.`id`,
`bookings`.`client_id`,
`bookings`.`vehicle_id`,
`bookings`.`vehicle_hire`,
(insurance_driver_1 + insurance_driver_2 + insurance_driver_3) AS insurance_total,
`bookings`.`bedding_qty`,
`bookings`.`bedding_price`,
`bookings`.`promo_discount`,
`bookings`.`promo_is_percent`,
`bookings`.`promo_code`,
(vehicle_hire + insurance_driver_1 + insurance_driver_2 + insurance_driver_3) + (bedding_qty * bedding_price) AS sub_total,
CASE
WHEN promo_is_percent = 1
THEN (((vehicle_hire + insurance_driver_1 + insurance_driver_2 + insurance_driver_3) + (bedding_qty * bedding_price)) / 100) * promo_discount
WHEN promo_is_percent = 0
THEN promo_discount
END as discount_total,
(sub_total - discount_total) AS total
FROM `bookings`
WHERE `bookings`.`status` = 'Quote'
ORDER BY `bookings`.`id` desc
LIMIT 0, 10
Thanks
Update
Working example below,
`SELECT SQL_CALC_FOUND_ROWS *,
(sub_total - discount_total) AS total
FROM (
SELECT
`bookings`.`id`,
`bookings`.`client_id`,
`bookings`.`vehicle_id`,
`bookings`.`vehicle_hire`,
(insurance_driver_1 + insurance_driver_2 + insurance_driver_3) AS insurance_total,
`bookings`.`bedding_qty`,
`bookings`.`bedding_price`,
`bookings`.`promo_discount`,
`bookings`.`promo_is_percent`,
`bookings`.`promo_code`,
(vehicle_hire + insurance_driver_1 + insurance_driver_2 + insurance_driver_3) + (bedding_qty * bedding_price) AS sub_total,
CASE
WHEN promo_is_percent = 1
THEN (((vehicle_hire + insurance_driver_1 + insurance_driver_2 + insurance_driver_3) + (bedding_qty * bedding_price)) / 100) * promo_discount
WHEN promo_is_percent = 0
THEN promo_discount
END as discount_total
FROM `bookings`
WHERE `bookings`.`status` = 'Quote'
) AS src
ORDER BY src.`id` desc
LIMIT 0, 10`

Answer 1

For your question I would solve it like so:
SELECT *,
(sub_total - discount_total) AS total
FROM (
SELECT SQL_CALC_FOUND_ROWS
`bookings`.`id`,
`bookings`.`client_id`,
`bookings`.`vehicle_id`,
`bookings`.`vehicle_hire`,
(insurance_driver_1 + insurance_driver_2 + insurance_driver_3) AS insurance_total,
`bookings`.`bedding_qty`,
`bookings`.`bedding_price`,
`bookings`.`promo_discount`,
`bookings`.`promo_is_percent`,
`bookings`.`promo_code`,
(vehicle_hire + insurance_driver_1 + insurance_driver_2 + insurance_driver_3) + (bedding_qty * bedding_price) AS sub_total,
CASE
WHEN promo_is_percent = 1
THEN (((vehicle_hire + insurance_driver_1 + insurance_driver_2 + insurance_driver_3) + (bedding_qty * bedding_price)) / 100) * promo_discount
WHEN promo_is_percent = 0
THEN promo_discount
END as discount_total
FROM `bookings`
WHERE `bookings`.`status` = 'Quote'
) AS src
ORDER BY src.`id` desc
LIMIT 0, 10
but this is only if you're definitely trying to avoid retyping the whole formula for your column.
have you also considered creating a calculated column in mysql?