SQL Server Team-based Development - Red Gate Software

Chapter 6: Reusing T-SQL Code
Turn to triggers when constraints are not practical
As we have seen, constraints are extremely useful in many simple cases. However, our
business rules are often more complex, and it is sometimes not possible or not practical
to use constraints. To demonstrate this point, let's add one more table, TeamMembers,
which references the Teams table through the TeamID column, as shown in Listing 6-28.
CREATE TABLE dbo.TeamMembers
(
TeamMemberID INT NOT NULL ,
TeamID INT NOT NULL ,
Name VARCHAR(50) NOT NULL ,
IsTeamLead CHAR(1) NOT NULL ,
CONSTRAINT PK_TeamMembers PRIMARY KEY ( TeamMemberID ) ,
CONSTRAINT FK_TeamMembers_Teams
FOREIGN KEY ( TeamID ) REFERENCES dbo.Teams ( TeamID ) ,
CONSTRAINT CHK_TeamMembers_IsTeamLead
CHECK ( IsTeamLead IN ( 'Y', 'N' ) )
) ;
Listing 6-28: Creating the TeamMembers table.
Suppose that we need to implement the following business rule: no team can have more
than two members. Implementing this business rule in a trigger is quite straightforward,
as shown in Listing 6-29, and you only have to do it once. It is possible, but much more
complex, to implement this rule via constraints.
CREATE TRIGGER dbo.TeamMembers_TeamSizeLimitTrigger
ON dbo.TeamMembers
FOR INSERT, UPDATE
AS
IF EXISTS ( SELECT *
FROM ( SELECT TeamID ,
TeamMemberID
FROM inserted
UNION
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