Calculate a value with other values - sql

Question

Let's make it simple:
USE Example1
CREATE TABLE Person
(PersonID int PRIMARY KEY IDENTITY(1,1),
FirstName nchar(20) NOT NULL,
LastName nchar(20) NOT NULL,
Salary money NOT NULL
)
CREATE TABLE Student
(StudentID int PRIMARY KEY IDENTITY(1,1),
FirstName nchar(20) NOT NULL,
LastName nchar(20) NOT NULL,
FatherID int NOT NULL,
MotherID int NOT NULL,
CONSTRAINT fk_Student_FatherID FOREIGN KEY (FatherID)
REFERENCES Person(PersonID),
CONSTRAINT fk_Student_MotherID FOREIGN KEY (MotherID)
REFERENCES Person(PersonID)
)
CREATE TABLE Registration
(RegistrationID int PRIMARY KEY IDENTITY(1,1),
StudentID int NOT NULL,
Date datetime NOT NULL,
MonthlyPayment ??????????
CONSTRAINT fk_Registration_StudentID FOREIGN KEY (StudentID)
REFERENCES Student(StudentID)
)
INSERT INTO Person VALUES ('John','Doe','1000')
INSERT INTO Person VALUES ('Mary','Poppins','800')
INSERT INTO Student VALUES ('Gary','Doe', 1, 2)
INSERT INTO Registration VALUES (1, getdate(),???)
I have a student that is going to make a registration in a school and have a monthly payment that is going do be FatherSalary*0.5 + MotherSalary*0.5 but I don't know how to make that happen. I'm new in SQL and maybe this is simple and I should know how to make it, but I don't and I need help.

Answer 1

Are you sure you need MonthlyPayment column in your table?
You can create table Registration without MonthlyPayment field and then create a view
create view vw_Registration
as
select
R.RegistrationID,
R.StudentID,
R.Date,
F.Salary * 0.5 + M.Salary * 0.5 as MonthlyPayment
from Registration as R
left outer join Student as S on S.StudentID = R.StudentID
left outer join Person as F on F.PersonID = S.FatherID
left outer join Person as M on M.PersonID = S.MotherId
SQL FIDDLE EXAMPLE

Answer 2

If the expression "FatherSalary*0.5 + MotherSalary*0.5" will not change, then you can use trigger i suppose.Your trigger will check the inserts to your Registration table. At the time of an insert, it will get the student id, then using this id it will get the necessary data from your student table and your Person table. At that moment, you will have access to the Money column for both father and mother. Calculate the result, and let the trigger to the insert for you.

Answer 3

A view sounds good at first hand, but I can imagine a scenario where you want to calculate a monthly payment based on the data available at a certain point in time so the monthly payment does not change everytime the father or mother has a change in salary...
In that case you can use this:
INSERT INTO Registration
(StudentID, Date, MonthlyPayment)
SELECT S.StudentID, getdate(), ISNULL(F.Salary, 0) * 0.5 + ISNULL(M.Salary, 0) * 0.5
FROM Student as S
left outer join Person as F on F.PersonID = S.FatherID
left outer join Person as M on M.PersonID = S.MotherId
WHERE S.StudentID = 1
SQL Fiddle