How do I create a multiple-table check constraint?

Question

Please imagine this small database...

Diagram

removed dead ImageShack link - volunteer database diagram

Tables

Volunteer     Event         Shift         EventVolunteer
=========     =====         =====         ==============
Id            Id            Id            EventId
Name          Name          EventId       VolunteerId
Email         Location      VolunteerId
Phone         Day           Description
Comment       Description   Start
                            End

Associations

Volunteers may sign up for multiple events.
Events may be staffed by multiple volunteers.

An event may have multiple shifts.
A shift belongs to only a single event.

A shift may be staffed by only a single volunteer.
A volunteer may staff multiple shifts.

Check Constraints

1. Can I create a check constraint to enforce that no shift is staffed by a volunteer that's not signed up for that shift's event?

2. Can I create a check constraint to enforce that two overlapping shifts are never staffed by the same volunteer?

Answer 1

The best place to enforce data integrity is the database. Rest assured that some developer, intentionally or not, will find a way to sneak inconsistent stuff into the database if you let them!

Here's an example with check constraints:

CREATE FUNCTION dbo.SignupMismatches()
RETURNS int
AS BEGIN RETURN (
    SELECT count(*)
    FROM Shift s
    LEFT JOIN EventVolunteer ev
    ON ev.EventId = s.EventId
    AND ev.VolunteerId = s.VolunteerId
    WHERE ev.Id is null
) END
go
ALTER TABLE Shift ADD CONSTRAINT chkSignup CHECK (dbo.SignupMismatches() = 0);
go
CREATE FUNCTION dbo.OverlapMismatches()
RETURNS int
AS BEGIN RETURN (
    SELECT count(*)
    FROM Shift a
    JOIN Shift b
    ON a.id <> b.id
    AND a.Start < b.[End]
    AND a.[End] > b.Start
    AND a.VolunteerId = b.VolunteerId
) END
go
ALTER TABLE Shift ADD CONSTRAINT chkOverlap CHECK (dbo.OverlapMismatches() = 0);

Here's some tests for the new data integrity checks:

insert into Volunteer (name) values ('Dubya')
insert into Event (name) values ('Build Wall Around Texas')

-- Dubya tries to build a wall, but Fails because he's not signed up
insert into Shift (VolunteerID, EventID, Description, Start, [End]) 
    values (1, 1, 'Dunbya Builds Wall', '2010-01-01', '2010-01-02')

-- Properly signed up?  Good
insert into EventVolunteer (VolunteerID, EventID) 
    values (1, 1)
insert into Shift (VolunteerID, EventID, Description, Start, [End]) 
    values (1, 1, 'Dunbya Builds Wall', '2010-01-01', '2010-01-03')

-- Fails, you can't start the 2nd wall before you finished the 1st
insert into Shift (VolunteerID, EventID, Description, Start, [End]) 
    values (1, 1, 'Dunbya Builds Second Wall', '2010-01-02', '2010-01-03')

Here are the table definitions:

set nocount on
if OBJECT_ID('Shift') is not null
    drop table Shift
if OBJECT_ID('EventVolunteer') is not null
    drop table EventVolunteer
if OBJECT_ID('Volunteer') is not null
    drop table Volunteer
if OBJECT_ID('Event') is not null
    drop table Event
if OBJECT_ID('SignupMismatches') is not null
    drop function SignupMismatches
if OBJECT_ID('OverlapMismatches') is not null
    drop function OverlapMismatches

create table Volunteer (
    id int identity primary key
,   name varchar(50)
)
create table Event (
    Id int identity primary key
,   name varchar(50)
)
create table Shift (
    Id int identity primary key
,   VolunteerId int foreign key references Volunteer(id)
,   EventId int foreign key references Event(id)
,   Description varchar(250)
,   Start datetime
,   [End] datetime
)
create table EventVolunteer (
    Id int identity primary key
,   VolunteerId int foreign key references Volunteer(id)
,   EventId int foreign key references Event(id)
,   Location varchar(250)
,   [Day] datetime
,   Description varchar(250)
)

Answer 2

Question 1 is easy. Just have your Shift table refer directly to EventVolunteer table and you are all set

Answer 3

What I would do is have an Identity column on the EventVolunteer table that auto-increments, with a unique constraint on the EventId, VolunteerId pair. Use the EventVolunteerId (identity) as the foreign key to the Shift table. This enforces the constraint you'd like fairly simply, whilst normalizing your data somewhat.

I understand this is not the answer to your general question, however I'd see this as the best solution to your specific problem.

Edit:

I should have read the question fully. This solution will prevent one volunteer from doing two shifts at the same event, even if they don't overlap. Perhaps moving the shift start and end times to the EventVolunteer and having the check constraint on times on that table would suffice, though then you have shift data outside the Shift table which does not sound intuitive to me.

Answer 4

There is a way to do it by using triggers, which i wouldn't recommend. I would recommend not putting your buisness logic at the database level. The db doesn't need to know who, is staffing a certain shift at which time. That logic should be put in your buisness layer. I would recommend using a repository construction pattern. Scott gutherie has a very good chapter in his mvc 1.0 book which describes this (Link below).

http://weblogs.asp.net/scottgu/archive/2009/03/10/free-asp-net-mvc-ebook-tutorial.aspx