SQL，2个表的一个字段的总和

Question

I have 2 tables, student and family .

Half of the students come Sundays, the other half Saturdays

Some families have more than one child.

Families pay fees for the school. Some families because of some critearia have a discount.

"discount" is a field in 'family' table

In a report I need to calculate the sum of all the discounts of all families that take advantage of a discount (by day).

When I do so, if a family has 2 children, the result returns double of the discount, if 3 children 3 x discount....

My Query:

$select_total_discounts = 
"SELECT SUM(Discount) AS total_discounts
FROM family, student
WHERE family.Family_ID=student.Family_ID
AND student.Day like '%$Day%'  ;" ; 

$query= mysql_query($select_total_discounts,$conn) ; 
$row = mysql_fetch_assoc($query);
$total_discounts= $row['total_discounts'];

Thank you in advance

Answer 1

If I understood your question correctly, you want to calculate the discounts by family disregarding the amount of students it has. So it'd be like this (change the day value by your variable):

SELECT SUM(Discount) AS total_discounts
FROM family 
WHERE family.Family_ID in (select student.Family_ID from student  where student.Day like 'Monday')

So it sums the discounts of the families that have at least one student from the specified day. If it has one, two, three or more students, it will not affect the result.

Working SQLFiddle: http://sqlfiddle.com/#!2/9b9793/1

Answer 2

It is not really full answer, but I posted it here, so it would be readable.

I think you are looking for something like UNION :

SELECT sum(value) FROM ((SELECT value FROM table) UNION ALL (SELECT value FROM table2)) sq

UNION merges your tables vertically.