Defensive Database Programming - Red Gate Software

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Chapter 7: Advanced Use of ConstraintsThe Ticket-Tracking SystemIn Chapter 4, we implemented a trigger-based solution to the problem of assigningtickets in a ticket-tracking system. Our AFTER UPDATE trigger, dbo.Developers_Upd,enforced the following two business rules:• developers cannot go on vacation if they have active tickets assigned to them• inactive tickets cannot be changed to active status if they are assigned to developersthat are on vacation.Here, we'll re-implement these business rules using only constraints and, in addition,we'll be able to enforce the rule that "newly added active tickets cannot be assigned toa developer on vacation," which would have required an additional AFTER INSERTtrigger. To ensure that we are all on the same page, we'll drop the original tables fromChapter 4 and start again from scratch.IF EXISTS ( SELECT *FROM sys.objectsWHERE OBJECT_ID= OBJECT_ID(N'dbo.Tickets') )BEGIN ;DROP TABLE dbo.Tickets ;END ;GOIF EXISTS ( SELECT *FROM sys.objectsWHERE OBJECT_ID= OBJECT_ID(N'dbo.Developers') )BEGIN ;DROP TABLE dbo.Developers ;END ;GOCREATE TABLE dbo.Developers(DeveloperID INT NOT NULL ,210
Chapter 7: Advanced Use of ConstraintsFirstName VARCHAR(30) NOT NULL ,Lastname VARCHAR(30) NOT NULL ,DeveloperStatus VARCHAR(8) NOT NULL ,CONSTRAINT PK_Developers PRIMARY KEY (DeveloperID) ,CONSTRAINT CHK_Developers_StatusCHECK (DeveloperStatusIN ( 'Active', 'Vacation' ) )) ;CREATE TABLE dbo.Tickets(TicketID INT NOT NULL ,AssignedToDeveloperID INT NULL ,Description VARCHAR(50) NOT NULL ,TicketStatus VARCHAR(6) NOT NULL ,DeveloperStatus VARCHAR(8) NOT NULL ,CONSTRAINT PK_Tickets PRIMARY KEY ( TicketID ) ,CONSTRAINT FK_Tickets_DevelopersFOREIGN KEY ( AssignedToDeveloperID )REFERENCES dbo.Developers ( DeveloperID ) ,CONSTRAINT CHK_Tickets_StatusCHECK ( TicketStatus IN ( 'Active', 'Closed' ) )) ;Listing 7-1: Recreating the Developers and Tickets tables.Enforcing business rules using constraints onlyThe Developers table is identical to one we used in Chapter 4, but the Tickets tableis different; it has an extra column, DeveloperStatus, which we will use in a CHECKconstraint that attempts to enforce both our business rules, as shown in Listing 7-2.211
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Defensive Database Programming - Red Gate Software

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