sql server trigger for insert new record - sql-server-2008

Question

i have a table called 'tblDive' with columns:
create table tblDive (
DiveNumber int
InstructorNumber int
ClubNumber int
InstructorSigniture date
)
and another table:
create table tblWorksAt (
InstructorNumber int
ClubNumber int
StartWorkingDate date
EndWorkingDate date
)
the table 'tblWorksAt' has this record:
InstructorNumber | ClubNumber | StartWorkingDate | EndWorkingDate
1 2 1.1.2000 1.1.2005
i want to create a trigger that checks when inserting a new dive, if the instructor really worked at this club in the same time of signing on the dive.
so for example if i insert a new dive:
insert into tblDive (DiveNumber InstructorNumber ClubNumber InstructorSigniture)
values 111, 1, 2, 1.1.2009
i won't be able to insert this record because instructor number 1 stopped working at club number 2 in 1.1.2005

Answer 1

An alternative to using a trigger is to use a check constraint and a user defined function.
A function that checks is the instructor is employed at the right club at the right time:
CREATE FUNCTION CheckEmployment(#InstructorNumber int, #ClubNumber int, #checkdate date)
RETURNS int
AS
BEGIN
DECLARE #retval int
SELECT #retval = COUNT(*)
FROM tblWorksAt
WHERE InstructorNumber = #InstructorNumber
AND ClubNumber = #ClubNumber
AND (EndWorkingDate IS NULL OR EndWorkingDate > #checkdate)
RETURN #retval
END;
GO
And a check constraint using it:
ALTER TABLE tblDive
ADD CONSTRAINT chkEmployed
CHECK (dbo.CheckEmployment(InstructorNumber, ClubNumber, InstructorSigniture) != 0);
This might not be the most efficient way, but it should get the job done. The logic in the function might need improvement too, I might have missed something.
Sample SQL Fiddle showing it in action.

Answer 2

What I'll do is give you some hints and not-so-obvious information about triggers that may help you write the trigger, but you need to write it.
The inserted table
In SQL Server triggers, there are 2 pseudo-tables that you can reference: inserted and deleted tables. The names are somewhat deceiving, particularly if you are doing an UPDATE. The thing to remember is that under the hood, an UPDATE is a delete plus insert.
So essentially, inserted is the new (or updated) rows and deleted is the deleted rows and/or previous rows from an UPDATE before the changes were applied.
For a straight INSERT statement, the deleted table ought to be empty.
Therefore, you want to look for rows in inserted that meet a certain set of criteria. There are two logical ways to approach this:
Check if all rows meet this condition
Look for any rows that do not meet the condition.
Join
If you join inserted to tblWorksAt, you now have all the data you need. Something like this to join the tables and find rows that pass your business rules:
select 1
from inserted i
inner join tblWorksAt wa on wa.InstructorNumber = i.InstructorNumber and i.ClubNumber = wa.ClubNumber
where i.InstructorSignature between wa.StartWorkingDate and wa.EndWorkingDate
What to do with the query
Like I said before:
Check if all rows meet this condition
Look for any rows that do not meet the condition.
To check if all rows pass that criteria, you could:
Check if the count of rows matching that query equals the number of total rows in inserted, without a join.
Loop over each row in inserted which I will tell you right now, is almost never a good idea in a trigger.
To check if at least one row fails to pass the criteria, you could:
Change the between to not between and wrap this query around if (exists(...)).
Change the inner join to left outer join, move the where clause to the join, and then add a new WHERE clause that says tblWorksAt.InstructorNumber is null, then wrap this query around if (exists(...)).
Throwing an error
Now you know how to find rows that pass or fail. Now you just need to throw an error to prevent the statement from completing and prevent the data from persisting. I will leave that as an exercise to you. It should be easy to research.