Defensive Database Programming - Red Gate Software

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Chapter 7: Advanced Use of ConstraintsALTER TABLE dbo.TicketsADD CONSTRAINT CHK_Tickets_ValidStatusesCHECK( (TicketStatus = 'Active'AND DeveloperStatus = 'Active')OR TicketStatus = 'Closed') ;Listing 7-2: The CHK_Tickets_ValidStatuses constraint enforces bothbusiness rules.However, this constraint makes a serious assumption, and we do not know yet ifwe can enforce it. Can we really guarantee that the Tickets.DeveloperStatuscolumn will always match the Developers.DeveloperStatus column, for theassigned developer?The answer is "maybe." FOREIGN KEY constraints are supposed to guarantee thatcolumns in different tables match and the one shown in Listing 7-3 attempts to do justthat. We'll discuss why we need the ON UPDATE CASCADE clause shortly.ALTER TABLE dbo.TicketsADD CONSTRAINT FK_Tickets_Developers_WithStatusFOREIGN KEY (AssignedToDeveloperID, DeveloperStatus)REFERENCES dbo.Developers( DeveloperID, DeveloperStatus )ON UPDATE CASCADE ;Listing 7-3: FK_Tickets_Developers_WithStatus attempts to ensure that,for a given developer, the relevant column values match.Unfortunately, when we run Listing 7-3, it fails. Fortunately, the error message is veryexplicit and clear.212
Chapter 7: Advanced Use of ConstraintsMsg 1776, Level 16, State 0, Line 1There are no primary or candidate keys in the referencedtable 'dbo.Developers' that match the referencing column listin the foreign key 'FK_Tickets_Developers_WithStatus'.Msg 1750, Level 16, State 0, Line 1Could not create constraint. See previous errors.Listing 7-4: A very clear error message.As the error states, the column, or combination of columns, to which a FOREIGN KEYrefers in the parent table, in this case (DeveloperID, DeveloperStatus), must beunique. The uniqueness can be enforced very easily using a UNIQUE constraint or, asshown in Listing 7-5, a UNIQUE index.CREATE UNIQUE INDEX UNQ_Developers_IDWithStatusON dbo.Developers( DeveloperID, DeveloperStatus ) ;Listing 7-5: Enforcing the uniqueness of (DeveloperID, DeveloperStatus) inthe Developers table.The reason I chose to use an index here, rather than a constraint, is because I try to usethe latter strictly to enforce business rules, and the former for performance or otherissues. In this case, we are not enforcing a business rule (as DeveloperID by itself isalready a candidate key) so much as overcoming a SQL Server technicality, and thereforeI chose to use an index.Now Listing 7-3 will complete successfully. Before testing our solution, let's summarizethe changes we have made so far.• Added a new column, DeveloperStatus, to the Tickets table.• Added a CHECK constraint, CHK_Tickets_ValidStatuses, to enforce ourbusiness rules.• Created a UNIQUE index on (DeveloperID, DeveloperStatus) in theDevelopers table, which allowed these two columns to be used as the parentcolumns in a foreign key relationship.213
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Chapter 2: Code Vulnerabilities due
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Enforcing Data Integrity in the App
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About the AuthorAlex Kuznetsov has
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IntroductionResilient T-SQL code is
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IntroductionSpecific examples demon
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Chapter 1: Basic Defensive Database
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Defensive Database Programming - Red Gate Software

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