wrong result mysql in function only - mysql

Question

I have the following function, mysql query:
BEGIN
DECLARE r float(10,2);
DECLARE var_total float(10,2);
DECLARE var_discount float(10,2) DEFAULT null;
SELECT
sum(x.amount)
FROM
(
(SELECT
student_booking_school_course_price as amount
FROM
tbl_student_booking_school_course
WHERE
student_booking_id=par_student_booking_id
)
UNION
(SELECT
student_booking_school_accommodation_price as amount
FROM
tbl_student_booking_school_accommodation
WHERE
student_booking_id=par_student_booking_id
)
UNION
(SELECT
student_booking_school_insurance_price as amount
FROM
tbl_student_booking_school_insurance
WHERE
student_booking_id=par_student_booking_id
)
UNION
(SELECT
student_booking_school_transfer_price as amount
FROM
tbl_student_booking_school_transfer
WHERE
student_booking_id=par_student_booking_id
)
) x
INTO var_total;
IF var_total IS NULL THEN
SET r = 0;
END IF;
-- discount
SET var_discount = (SELECT
sb.student_booking_discount_amount
FROM
tbl_student_booking sb
WHERE
sb.student_booking_id=par_student_booking_id LIMIT 1);
IF var_discount IS NOT NULL THEN
SET r = var_total - var_discount;
end if;
return r;
END
The values are:
9698.88 course
559.55 accommodation
559.55 insurance
145.98 discount
It seems that the first query inside the function, only sums distinct values, as the result with discount is: 10112.45, so is not summing one value of 559.55, I tried to output different things as concat with a string and only see the result as 9698.88course,559.55accommodation, etc.. and it is fine. So I assume the issue is that is not summing if values are equals. The strange thing is that running this from the console, only the query outside the function, it sums ok.
My question is this a normal behaviour of MySql?If so is there a way to prevent this? is this a bug?

Answer 1

What you need here is UNION ALL clause:
SELECT
sum(x.amount)
FROM
(
(SELECT
student_booking_school_course_price as amount
FROM
tbl_student_booking_school_course
WHERE
student_booking_id=par_student_booking_id
)
UNION ALL
(SELECT
student_booking_school_accommodation_price as amount
FROM
tbl_student_booking_school_accommodation
WHERE
student_booking_id=par_student_booking_id
)
UNION ALL
(SELECT
student_booking_school_insurance_price as amount
FROM
tbl_student_booking_school_insurance
WHERE
student_booking_id=par_student_booking_id
)
UNION ALL
(SELECT
student_booking_school_transfer_price as amount
FROM
tbl_student_booking_school_transfer
WHERE
student_booking_id=par_student_booking_id
)
) x
INTO var_total;
The MySQL UNION Documentation says:
A DISTINCT union can be produced explicitly by using UNION DISTINCT or
implicitly by using UNION with no following DISTINCT or ALL keyword.