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         


        

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)
)